feat(client): district morphology generation overlay in the Atlas (T-1046, D-226)

Client half — the cascade's district tier is now visible in the Atlas map.

- protocol.gd: decode the district_grid field from AtlasLayerResponse.
- atlas_viewer.gd: a gen_district ('MRPH') toggle in OVERLAY_DEFS + the
  district-grid generation state (set/get) + wire it from the Ready response.
- atlas_marker_overlay.gd: _draw_gen_district() paints the coarse cols×rows grid,
  each cell coloured by its MorphologyZone discriminant (D-239 §6, 17-zone
  palette), semi-transparent under the Layer-1 line overlays. Planetary map view
  (not the 2km on-demand districts — those are Phase 5 in-world).

gdUnit4 test: overlay registered + district grid round-trips through the viewer
and the protocol decode. Visual tuning of the palette can follow once eyeballed
in the running Atlas.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

client/tests/test_atlas_overlays.gd

@@ -9,7 +9,29 @@ func test_generation_overlays_registered() -> void:
 	var ids: Array = []
 	for d: Dictionary in AtlasViewer.OVERLAY_DEFS:
 		ids.append(d["id"])
-	assert_that(ids).contains(["gen_l1_rivers", "gen_l1_basins", "gen_l1_attractors"])
+	assert_that(ids).contains(
+		["gen_l1_rivers", "gen_l1_basins", "gen_l1_attractors", "gen_district"]
+	)
+
+
+## T-1046: the district/morphology grid round-trips through the viewer and the
+## protocol decode surfaces it; referencing AtlasMarkerOverlay compiles the draw.
+func test_district_grid_round_trips() -> void:
+	var v: AtlasViewer = auto_free(AtlasViewer.new())
+	assert_that(v.get_generation_district_grid()).is_null()
+	var grid := {
+		"cols": 2,
+		"rows": 1,
+		"morphology": PackedByteArray([8, 14]),
+		"elev_q": PackedByteArray([10, 90]),
+	}
+	v.set_generation_district_grid(grid)
+	assert_that(v.get_generation_district_grid()).is_equal(grid)
+	# The decoded response dict carries the district_grid key (protocol.gd, T-1046).
+	var decoded: Variant = Protocol.atlas_response_from_raw(
+		{"body_id": "GJ1c", "status": "Ready", "district_grid": grid}
+	)
+	assert_that((decoded as Dictionary).get("district_grid")).is_equal(grid)
 
 
 func test_generation_data_round_trips() -> void: