fix(client): address PR #30 review — 9 items from Hoshe and Tyre

- Add SimBridge._last_snapshot = null to P3 before_test() (contamination risk)
- Clarify mono.is_visible is custom property, not CanvasItem builtin
- Camera smoothing test uses approximate equality instead of exact
- Propagate inner _encode_message errors in array/dict encoding
- Add explanatory comment on int_64 encoder branch
- Document unfalsifiable gauntlet guard as intentional future-proof
- Remove loose D-053 citation from LERP_SPEED pin
- Assert FogOverlay is sibling of FogGroup under World
- Add CANVAS_MODAL=30 (ModalLayer) assertion in P3-Z04

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

client/tests/test_client_p2.gd

@@ -80,7 +80,10 @@ func test_camera_smoothing_convergence() -> void:
 	for i in 5:
 		inst._process(0.016)
 	var expected := GameState.player_position * Constants.TILE_SIZE
-	assert_that(camera.global_position).is_equal(expected)
+	var dist := camera.global_position.distance_to(expected)
+	assert_that(dist < 0.1).override_failure_message(
+		"Camera should converge to player position (dist: %.4f)" % dist
+	).is_true()
 
 
 func test_camera_viewport_tracks_player_position() -> void:
@@ -191,6 +194,10 @@ func test_entity_player_color_regardless_of_sector() -> void:
 
 func test_monologue_display_visible_hidden() -> void:
 	# P2-U01: MonologueDisplay starts hidden, becomes visible after show_monologue.
+	# Note: mono.is_visible is a custom bool property on MonologueDisplay
+	# (monologue_display.gd:11), not the built-in CanvasItem.is_visible() method.
+	# The monologue uses tween alpha for visual hide/show, so the built-in
+	# .visible stays true — we test the script's own state tracking.
 	var inst := _make_scene()
 	var mono = inst.get_node("UILayer/MonologueDisplay")
 	assert_that(mono.is_visible).override_failure_message(