fix(client): isolate viewer tests from the machine-shared atlas disk cache

user://atlas_cache/ is one directory for every worktree gate run, live
capture driver, and real play session, and T-1183's Tier-2/3 lookup
short-circuits BEFORE test_mode's silent-no-op IPC — so a warm shared
cache delivers real canvases into tests written against 'nothing ever
arrives'. Caught live: a concurrent GJ380c Global capture flipped the
two before-any-canvas viewer tests in another worktree's push gate.

Adds disk_cache_root_override on StepCanvasViewer (threads into
StepCanvasRequest's existing test-injection-only seam; production
leaves it empty) and routes every direct viewer construction in the
viewer + legend suites through a _make_viewer() helper pinning an
isolated root. First slice of T-1193's viewer test seam; screen-built
viewers still share the default root (noted on T-1193).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

client/tests/test_step_canvas_legend.gd

@@ -9,9 +9,20 @@ extends GdUnitTestSuite
 const LegendScript := preload("res://ui/implant/apps/atlas/step_canvas/step_canvas_legend.gd")
 const StepCanvasTransport := preload("res://ui/implant/apps/atlas/step_canvas/step_canvas_transport.gd")
 
+## Same Tier-2/3 isolation as test_step_canvas_viewer.gd's _make_viewer()
+## (T-1193 first slice): the default disk-cache root is machine-shared, and a
+## warm cache delivers canvases into tests entered on real body ids.
+const TEST_DISK_CACHE_ROOT := "user://test_step_canvas_legend_cache/"
+
+
+func _make_viewer() -> StepCanvasViewer:
+	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	v.disk_cache_root_override = TEST_DISK_CACHE_ROOT
+	return v
+
 
 func test_legend_starts_hidden_before_refresh() -> void:
-	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	var v: StepCanvasViewer = _make_viewer()
 	add_child(v)
 	var legend = LegendScript.new(v)
 	auto_free(legend)
@@ -19,7 +30,7 @@ func test_legend_starts_hidden_before_refresh() -> void:
 
 
 func test_refresh_makes_the_legend_visible_and_populates_content() -> void:
-	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	var v: StepCanvasViewer = _make_viewer()
 	add_child(v)
 	v.enter({"body_id": "GJ380c", "body_radius_km": 6238.4}, {})
 	var legend = LegendScript.new(v)
@@ -45,7 +56,7 @@ func test_refresh_without_a_viewer_does_not_crash() -> void:
 ## default, one section per active toggle" contract the retired
 ## atlas_window_legend.gd's own refresh() doc described.
 func test_refresh_reflects_the_active_overlay_toggle() -> void:
-	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	var v: StepCanvasViewer = _make_viewer()
 	add_child(v)
 	v.enter({"body_id": "GJ380c", "body_radius_km": 6238.4}, {})
 	var legend = LegendScript.new(v)
@@ -63,7 +74,7 @@ func test_refresh_reflects_the_active_overlay_toggle() -> void:
 
 
 func test_reposition_sets_a_fixed_panel_margin_position() -> void:
-	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	var v: StepCanvasViewer = _make_viewer()
 	add_child(v)
 	var legend = LegendScript.new(v)
 	auto_free(legend)
@@ -81,7 +92,7 @@ func test_reposition_sets_a_fixed_panel_margin_position() -> void:
 ## column, with room to spare — never right up against it, and certainly
 ## never past it into where the canvas is centered from.
 func test_legend_actual_right_edge_stays_inside_the_reserved_column() -> void:
-	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	var v: StepCanvasViewer = _make_viewer()
 	add_child(v)
 	var legend = LegendScript.new(v)
 	auto_free(legend)

client/tests/test_step_canvas_viewer.gd

@@ -16,16 +16,34 @@ const StepCanvasTransport := preload("res://ui/implant/apps/atlas/step_canvas/st
 ## .cache/screenshots/t1183-eyeball-run2/02-region.png.
 const GJ1C_GLOBAL_EXTENT := Vector2i(177, 88)
 
+## Isolated Tier-2/3 disk-cache root for every directly-constructed viewer
+## (T-1193 first slice). The default user://atlas_cache/ is SHARED across the
+## whole machine — every worktree's gate run, live capture driver, and real
+## play session writes the same directory — and T-1183's disk-cache lookup
+## short-circuits BEFORE test_mode's silent-no-op IPC, so a warm shared cache
+## delivers real canvases into tests written against "nothing ever arrives"
+## (2026-07-25: a concurrent live GJ380c Global capture flipped the two
+## before-any-canvas tests in another worktree's gate run). In test_mode the
+## isolated root stays empty forever: no canvas ever arrives, so the cache
+## never writes.
+const TEST_DISK_CACHE_ROOT := "user://test_step_canvas_viewer_cache/"
+
+
+func _make_viewer() -> StepCanvasViewer:
+	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	v.disk_cache_root_override = TEST_DISK_CACHE_ROOT
+	return v
+
 
 func test_enter_lands_on_the_global_opener() -> void:
-	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	var v: StepCanvasViewer = _make_viewer()
 	add_child(v)
 	v.enter({"body_id": "GJ380c", "body_radius_km": 6238.4}, {})
 	assert_str(v.get_held_rung()).is_equal(StepCanvasTransport.RUNG_GLOBAL)
 
 
 func test_get_body_id_reflects_the_entered_body() -> void:
-	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	var v: StepCanvasViewer = _make_viewer()
 	add_child(v)
 	assert_str(v.get_body_id()).is_equal("")
 	v.enter({"body_id": "GJ380c", "body_radius_km": 6238.4}, {})
@@ -35,7 +53,7 @@ func test_get_body_id_reflects_the_entered_body() -> void:
 ## Scrolling one notch descends the ladder — cursor-anchored, so a cursor
 ## position must be supplied; the rung index advances by exactly one.
 func test_scroll_rung_descends_one_notch() -> void:
-	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	var v: StepCanvasViewer = _make_viewer()
 	add_child(v)
 	v.enter({"body_id": "GJ380c", "body_radius_km": 6238.4}, {})
 	v._scroll_rung(1, Vector2(400.0, 300.0))
@@ -43,7 +61,7 @@ func test_scroll_rung_descends_one_notch() -> void:
 
 
 func test_scroll_rung_clamps_at_the_deepest_rung() -> void:
-	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	var v: StepCanvasViewer = _make_viewer()
 	add_child(v)
 	v.enter({"body_id": "GJ380c", "body_radius_km": 6238.4}, {})
 	for _i in range(10):
@@ -52,7 +70,7 @@ func test_scroll_rung_clamps_at_the_deepest_rung() -> void:
 
 
 func test_reset_to_global_returns_from_a_deep_rung() -> void:
-	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	var v: StepCanvasViewer = _make_viewer()
 	add_child(v)
 	v.enter({"body_id": "GJ380c", "body_radius_km": 6238.4}, {})
 	v._scroll_rung(1, Vector2(400.0, 300.0))
@@ -71,7 +89,7 @@ func test_reset_to_global_returns_from_a_deep_rung() -> void:
 ## regression (scroll_step() clamping at index 0 meant _scroll_rung()
 ## returned before ever reaching a reset call).
 func test_scroll_out_at_global_after_a_pan_resets_the_view() -> void:
-	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	var v: StepCanvasViewer = _make_viewer()
 	add_child(v)
 	v.enter({"body_id": "GJ380c", "body_radius_km": 6238.4}, {})
 	assert_str(v.get_held_rung()).is_equal(StepCanvasTransport.RUNG_GLOBAL)
@@ -94,7 +112,7 @@ func test_scroll_out_at_global_after_a_pan_resets_the_view() -> void:
 ## (un-drifted) Global frame must stay a no-op — the reset is edge-triggered
 ## on genuine drift, not a per-scroll unconditional reset.
 func test_scroll_out_at_undrifted_global_is_a_no_op() -> void:
-	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	var v: StepCanvasViewer = _make_viewer()
 	add_child(v)
 	v.enter({"body_id": "GJ380c", "body_radius_km": 6238.4}, {})
 	v._scroll_rung(-1, Vector2(400.0, 300.0))
@@ -103,21 +121,21 @@ func test_scroll_out_at_undrifted_global_is_a_no_op() -> void:
 
 
 func test_overlay_visibility_defaults_to_off_for_every_toggle() -> void:
-	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	var v: StepCanvasViewer = _make_viewer()
 	add_child(v)
 	for def: Dictionary in v.get_overlay_defs():
 		assert_bool(v.is_overlay_visible(def["id"])).is_false()
 
 
 func test_set_overlay_visible_updates_state() -> void:
-	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	var v: StepCanvasViewer = _make_viewer()
 	add_child(v)
 	v.set_overlay_visible("gen_dw_temp", true)
 	assert_bool(v.is_overlay_visible("gen_dw_temp")).is_true()
 
 
 func test_set_overlay_visible_unknown_id_is_a_no_op() -> void:
-	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	var v: StepCanvasViewer = _make_viewer()
 	add_child(v)
 	v.set_overlay_visible("not_a_real_overlay", true)
 	assert_bool(v.is_overlay_visible("not_a_real_overlay")).is_false()
@@ -201,7 +219,7 @@ static func _synthetic_canvas(width: int, height: int) -> Dictionary:
 
 
 func test_maybe_refloat_is_inert_before_any_canvas_has_arrived() -> void:
-	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	var v: StepCanvasViewer = _make_viewer()
 	add_child(v)
 	v.enter({"body_id": "GJ380c", "body_radius_km": 6238.4}, {})
 	v._scroll_rung(1, Vector2(400.0, 300.0))  # District — footprint still ZERO, nothing arrived
@@ -214,7 +232,7 @@ func test_maybe_refloat_is_inert_before_any_canvas_has_arrived() -> void:
 ## the held canvas keeps drawing, no re-request (§4/§5's "only when a pan
 ## carries the view past the held window's edge").
 func test_maybe_refloat_does_not_refloat_on_a_small_pan() -> void:
-	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	var v: StepCanvasViewer = _make_viewer()
 	add_child(v)
 	v.size = Vector2(800.0, 600.0)  # a real viewport size — _maybe_refloat's
 	# drift math is relative to get_rect().size's own center; leaving this at
@@ -244,7 +262,7 @@ func test_maybe_refloat_does_not_refloat_on_a_small_pan() -> void:
 ## world_center, view_offset reset to ZERO (the canvas re-centers under the
 ## new request).
 func test_maybe_refloat_refloats_once_the_pan_crosses_the_edge_threshold() -> void:
-	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	var v: StepCanvasViewer = _make_viewer()
 	add_child(v)
 	v.enter({"body_id": "GJ380c", "body_radius_km": 6238.4}, {})
 	v._scroll_rung(1, Vector2(400.0, 300.0))  # District
@@ -266,7 +284,7 @@ func test_maybe_refloat_refloats_once_the_pan_crosses_the_edge_threshold() -> vo
 ## Global never re-floats on pan (D-255(a): its canvas is the whole body,
 ## no edge to cross) — even with a real texture held and a huge drift.
 func test_maybe_refloat_is_a_no_op_at_global_rung() -> void:
-	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	var v: StepCanvasViewer = _make_viewer()
 	add_child(v)
 	v.enter({"body_id": "GJ380c", "body_radius_km": 6238.4}, {})
 	v._terrain_layer.rebuild_from_canvas(_synthetic_canvas(200, 100), v.get_held_rung(), "")
@@ -284,7 +302,7 @@ func test_maybe_refloat_is_a_no_op_at_global_rung() -> void:
 
 
 func test_edge_scroll_suppressed_without_application_focus() -> void:
-	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	var v: StepCanvasViewer = _make_viewer()
 	add_child(v)
 	v.size = Vector2(800.0, 600.0)
 	v._app_has_focus = false
@@ -293,7 +311,7 @@ func test_edge_scroll_suppressed_without_application_focus() -> void:
 
 
 func test_edge_scroll_suppressed_when_cursor_has_never_moved_over_the_control() -> void:
-	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	var v: StepCanvasViewer = _make_viewer()
 	add_child(v)
 	v.size = Vector2(800.0, 600.0)
 	# _last_mouse_pos defaults to (-1, -1) — an impossible in-bounds position,
@@ -303,7 +321,7 @@ func test_edge_scroll_suppressed_when_cursor_has_never_moved_over_the_control()
 
 
 func test_edge_scroll_active_near_the_left_edge() -> void:
-	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	var v: StepCanvasViewer = _make_viewer()
 	add_child(v)
 	v.size = Vector2(800.0, 600.0)
 	v._app_has_focus = true
@@ -315,7 +333,7 @@ func test_edge_scroll_active_near_the_left_edge() -> void:
 
 
 func test_edge_scroll_active_near_the_right_edge() -> void:
-	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	var v: StepCanvasViewer = _make_viewer()
 	add_child(v)
 	v.size = Vector2(800.0, 600.0)
 	v._app_has_focus = true
@@ -325,7 +343,7 @@ func test_edge_scroll_active_near_the_right_edge() -> void:
 
 
 func test_edge_scroll_inactive_well_inside_the_viewport() -> void:
-	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	var v: StepCanvasViewer = _make_viewer()
 	add_child(v)
 	v.size = Vector2(800.0, 600.0)
 	v._app_has_focus = true
@@ -345,7 +363,7 @@ func test_edge_scroll_inactive_well_inside_the_viewport() -> void:
 ## ever cached under it, so the sweep is a true no-op read (no writes to the
 ## real user://atlas_cache/ directory this test could leak).
 func test_enter_runs_the_disk_cache_visit_sweep_without_crashing() -> void:
-	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	var v: StepCanvasViewer = _make_viewer()
 	add_child(v)
 	v.enter({"body_id": "T1183_sweep_smoke_test_body", "body_radius_km": 6238.4}, {})
 	# If the wiring is broken (e.g. calling a method that doesn't exist), the
@@ -358,7 +376,7 @@ func test_enter_runs_the_disk_cache_visit_sweep_without_crashing() -> void:
 ## Timer node, not a _process()-driven counter), autostarts, and is set to
 ## the documented coarse interval — never a sub-frame or per-frame value.
 func test_disk_sweep_timer_is_coarse_and_autostarts() -> void:
-	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	var v: StepCanvasViewer = _make_viewer()
 	add_child(v)
 	var timer: Timer = v.get_node("DiskSweepTimer")
 	assert_object(timer).is_not_null()
@@ -383,7 +401,7 @@ func test_disk_sweep_timer_is_coarse_and_autostarts() -> void:
 ## non-blocking test speed) against an injected-root request so this test
 ## touches no real cache files.
 func test_disk_sweep_timeout_handler_runs_background_sweep_for_the_current_body() -> void:
-	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	var v: StepCanvasViewer = _make_viewer()
 	add_child(v)
 	v.enter({"body_id": "T1183_sweep_smoke_test_body", "body_radius_km": 6238.4}, {})
 	# No live server, no cached entries for this body — the assertion is
@@ -422,7 +440,7 @@ static func _land_global_canvas(v: StepCanvasViewer, width: int, height: int) ->
 
 
 func test_global_canvas_arrival_populates_the_body_extent_cap_source() -> void:
-	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	var v: StepCanvasViewer = _make_viewer()
 	add_child(v)
 	v.enter({"body_id": "T1189_extent_letterbox_test_body", "body_radius_km": 6371.0}, {})
 	TestStepCanvasViewer._land_global_canvas(v, GJ1C_GLOBAL_EXTENT.x, GJ1C_GLOBAL_EXTENT.y)
@@ -434,7 +452,7 @@ func test_global_canvas_arrival_populates_the_body_extent_cap_source() -> void:
 ## scrolling to Region and firing its request must produce a CAPPED extent
 ## — never the raw viewport-fit 384x216 that overran the body on both axes.
 func test_region_request_extent_is_capped_to_the_landed_global_extent() -> void:
-	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	var v: StepCanvasViewer = _make_viewer()
 	add_child(v)
 	v.size = Vector2(1920.0, 1080.0)  # the T-1183 eyeball's own viewport
 	v.enter({"body_id": "T1189_extent_letterbox_test_body", "body_radius_km": 6371.0}, {})
@@ -457,7 +475,7 @@ func test_region_request_extent_is_capped_to_the_landed_global_extent() -> void:
 ## ZERO — the Region request must go out UNCAPPED (server clamps
 ## independently) rather than silently collapsing to a zero-cell request.
 func test_region_request_extent_is_uncapped_before_the_global_echo_lands() -> void:
-	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	var v: StepCanvasViewer = _make_viewer()
 	add_child(v)
 	v.size = Vector2(1920.0, 1080.0)
 	v.enter({"body_id": "T1189_extent_letterbox_test_body", "body_radius_km": 6371.0}, {})
@@ -481,7 +499,7 @@ func test_region_request_extent_is_uncapped_before_the_global_echo_lands() -> vo
 ## same key, capped or not, never a key built from one extent and served
 ## under another.
 func test_capped_extent_matches_what_the_request_actually_sends() -> void:
-	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	var v: StepCanvasViewer = _make_viewer()
 	add_child(v)
 	v.size = Vector2(1920.0, 1080.0)
 	v.enter({"body_id": "T1189_extent_letterbox_test_body", "body_radius_km": 6371.0}, {})
@@ -515,7 +533,7 @@ func test_capped_extent_matches_what_the_request_actually_sends() -> void:
 ## proving the cap is spacing-keyed, not applied indiscriminately to every
 ## rung once a Global extent is known.
 func test_district_request_extent_is_never_capped_by_the_global_extent() -> void:
-	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	var v: StepCanvasViewer = _make_viewer()
 	add_child(v)
 	v.size = Vector2(1920.0, 1080.0)
 	v.enter({"body_id": "T1189_extent_letterbox_test_body", "body_radius_km": 6371.0}, {})
@@ -538,7 +556,7 @@ func test_district_request_extent_is_never_capped_by_the_global_extent() -> void
 ## region grid") — this proves it, through the real enter()/land/enter()
 ## sequence rather than asserting the field directly only.
 func test_global_body_extent_resets_on_a_different_body_and_does_not_leak() -> void:
-	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	var v: StepCanvasViewer = _make_viewer()
 	add_child(v)
 	v.size = Vector2(1920.0, 1080.0)
 
@@ -575,7 +593,7 @@ func test_global_body_extent_resets_on_a_different_body_and_does_not_leak() -> v
 ## have computed a non-zero centering offset (unless the canvas happens to
 ## exactly fill the viewport, not the case here).
 func test_global_canvas_arrival_centers_the_view_not_top_left() -> void:
-	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	var v: StepCanvasViewer = _make_viewer()
 	add_child(v)
 	v.size = Vector2(1920.0, 1080.0)
 	v.enter({"body_id": "T1189_extent_letterbox_test_body", "body_radius_km": 6371.0}, {})
@@ -596,7 +614,7 @@ func test_global_canvas_arrival_centers_the_view_not_top_left() -> void:
 ## legend-reserved 1920x1080 case (R=9, scale=1.8) AND the 4K case (R=20,
 ## scale=4.0) — the SAME formula, no separate coverage-threshold branch.
 func test_global_canvas_scale_yields_an_exact_integer_pixels_per_gridunit_ratio() -> void:
-	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	var v: StepCanvasViewer = _make_viewer()
 	add_child(v)
 	v.size = Vector2(1920.0, 1080.0)
 	v.enter({"body_id": "T1189_extent_letterbox_test_body", "body_radius_km": 6371.0}, {})
@@ -618,7 +636,7 @@ func test_global_canvas_scale_yields_an_exact_integer_pixels_per_gridunit_ratio(
 ## too (20/5 = 4.0) — confirms the 1080p case above isn't a coincidence of
 ## a small viewport, just the same formula at a different achievable ratio.
 func test_global_canvas_scale_at_4k_also_yields_an_exact_integer_ratio() -> void:
-	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	var v: StepCanvasViewer = _make_viewer()
 	add_child(v)
 	v.size = Vector2(3840.0, 2160.0)
 	v.enter({"body_id": "T1189_extent_letterbox_test_body", "body_radius_km": 6371.0}, {})
@@ -633,7 +651,7 @@ func test_global_canvas_scale_at_4k_also_yields_an_exact_integer_ratio() -> void
 ## stays 1.0 once the player has descended past Global, even though a
 ## Global canvas was landed earlier in the same session.
 func test_fixed_rung_canvas_scale_stays_one_after_descending_from_global() -> void:
-	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	var v: StepCanvasViewer = _make_viewer()
 	add_child(v)
 	v.size = Vector2(1920.0, 1080.0)
 	v.enter({"body_id": "T1189_extent_letterbox_test_body", "body_radius_km": 6371.0}, {})
@@ -654,7 +672,7 @@ func test_fixed_rung_canvas_scale_stays_one_after_descending_from_global() -> vo
 ## and centered, its drawn rect's LEFT edge must be at or past the legend's
 ## own right edge, never underneath it.
 func test_global_canvas_left_edge_never_overlaps_the_legend_column() -> void:
-	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	var v: StepCanvasViewer = _make_viewer()
 	add_child(v)
 	v.size = Vector2(1920.0, 1080.0)
 	v.enter({"body_id": "T1189_extent_letterbox_test_body", "body_radius_km": 6371.0}, {})
@@ -671,7 +689,7 @@ func test_global_canvas_left_edge_never_overlaps_the_legend_column() -> void:
 ## one — _notification(NOTIFICATION_RESIZED) wires _recompute_canvas_transform()
 ## for exactly this case.
 func test_resize_recenters_an_already_held_global_canvas() -> void:
-	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	var v: StepCanvasViewer = _make_viewer()
 	add_child(v)
 	v.size = Vector2(1920.0, 1080.0)
 	v.enter({"body_id": "T1189_extent_letterbox_test_body", "body_radius_km": 6371.0}, {})
@@ -695,7 +713,7 @@ func test_resize_recenters_an_already_held_global_canvas() -> void:
 ## under 1.0, end-to-end through the real viewer wiring, not just the pure
 ## transport function this mirrors.
 func test_global_canvas_scale_never_drops_below_native_on_a_narrow_viewport() -> void:
-	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	var v: StepCanvasViewer = _make_viewer()
 	add_child(v)
 	v.size = Vector2(348.0 + StepCanvasTransport.LEGEND_COLUMN_PX, 1080.0)
 	v.enter({"body_id": "T1189_extent_letterbox_test_body", "body_radius_km": 6371.0}, {})
@@ -720,7 +738,7 @@ func test_global_canvas_scale_never_drops_below_native_on_a_narrow_viewport() ->
 ## (verified end-to-end, through act(), in test_atlas_agent_interface.gd;
 ## this is the narrower unit-level check directly against the viewer).
 func test_jump_to_sets_held_rung_and_world_center() -> void:
-	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	var v: StepCanvasViewer = _make_viewer()
 	add_child(v)
 	v.enter({"body_id": "GJ380c", "body_radius_km": 6238.4}, {})
 
@@ -733,7 +751,7 @@ func test_jump_to_sets_held_rung_and_world_center() -> void:
 ## Omitting `rung` keeps whatever rung is currently held — the "revisit
 ## within the same rung" common case shouldn't require repeating it.
 func test_jump_to_keeps_current_rung_when_omitted() -> void:
-	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	var v: StepCanvasViewer = _make_viewer()
 	add_child(v)
 	v.enter({"body_id": "GJ380c", "body_radius_km": 6238.4}, {})
 	v._scroll_rung(1, Vector2(400.0, 300.0))  # Region
@@ -746,7 +764,7 @@ func test_jump_to_keeps_current_rung_when_omitted() -> void:
 ## Jumping to Global always forces world_center to ZERO — Global has no
 ## panned-center concept (mirrors _scroll_rung()'s own Global-rung handling).
 func test_jump_to_global_forces_world_center_to_zero() -> void:
-	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	var v: StepCanvasViewer = _make_viewer()
 	add_child(v)
 	v.enter({"body_id": "GJ380c", "body_radius_km": 6238.4}, {})
 	v._scroll_rung(1, Vector2(400.0, 300.0))  # Region
@@ -757,7 +775,7 @@ func test_jump_to_global_forces_world_center_to_zero() -> void:
 
 
 func test_jump_to_unrecognized_rung_is_a_no_op() -> void:
-	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	var v: StepCanvasViewer = _make_viewer()
 	add_child(v)
 	v.enter({"body_id": "GJ380c", "body_radius_km": 6238.4}, {})
 	var rung_before: String = v.get_held_rung()
@@ -770,7 +788,7 @@ func test_jump_to_unrecognized_rung_is_a_no_op() -> void:
 
 
 func test_get_current_canvas_summary_before_any_canvas_arrives() -> void:
-	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	var v: StepCanvasViewer = _make_viewer()
 	add_child(v)
 	v.enter({"body_id": "GJ380c", "body_radius_km": 6238.4}, {})
 
@@ -786,7 +804,7 @@ func test_get_current_canvas_summary_before_any_canvas_arrives() -> void:
 ## StepCanvasAnnotationLayer._draw_settlements()'s own dedup discipline —
 ## covering the same cell id twice must not double-count).
 func test_get_current_canvas_summary_counts_match_a_fixture_canvas() -> void:
-	var v: StepCanvasViewer = auto_free(StepCanvasViewer.new())
+	var v: StepCanvasViewer = _make_viewer()
 	add_child(v)
 	v.enter({"body_id": "GJ380c", "body_radius_km": 6238.4}, {})
 	v._scroll_rung(1, Vector2(400.0, 300.0))  # Region

client/ui/implant/apps/atlas/step_canvas/step_canvas_viewer.gd

@@ -150,6 +150,14 @@ var _legend_panel = null
 var _request = null  # StepCanvasRequest
 var _disk_sweep_timer: Timer = null  # T-1183 coarse background sweep trigger
 
+## Test-injection-only (T-1193 first slice): threads into StepCanvasRequest's
+## disk_cache_root seam so tests can isolate the Tier-2/3 disk cache from the
+## machine-shared user://atlas_cache/ (one directory for every worktree gate,
+## live capture driver, and real play session — a warm shared cache delivers
+## canvases into tests written against a cold one). Set BEFORE add_child();
+## the request is constructed in _ready(). Production always leaves this "".
+var disk_cache_root_override: String = ""
+
 
 func _ready() -> void:
 	anchor_right = 1.0
@@ -176,7 +184,7 @@ func _ready() -> void:
 	_annotation_layer.name = "AnnotationLayer"
 	_canvas.add_child(_annotation_layer)
 
-	_request = StepCanvasRequest.new(self)
+	_request = StepCanvasRequest.new(self, disk_cache_root_override)
 	_request.name = "Request"
 	add_child(_request)
 	_request.canvas_ready.connect(_on_canvas_ready)