feat(world): seabed flora and encrusting growth on rock

SeabedFloraActor scatters seagrass tufts on flat sand in the lit band and
sea fans on rock between 15 and 60 m, deterministically from the site id,
one MultiMesh per tile and kind, with procedural alpha-cut meshes and
textures (SeabedFloraAssets), vertex sway and wrapped lighting, no
collision and no sonar presence. Placement reads the field arrays
directly and runs on the worker thread pool. The seabed shader grows
turf algae on sunlit rock, coralline crusts in patches and encrusting
sponges in the shade, fading out between 40 and 75 m. The water-column
optics move to a shared shader include.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

game/domain/sites/terrain_height_field.gd

@@ -64,6 +64,11 @@ func elevations_m() -> PackedFloat32Array:
 	return _data.elevations_m
 
 
+## Row-major sonar materials, same layout as elevations_m.
+func materials() -> PackedByteArray:
+	return _data.materials
+
+
 func elevation_at_cell(column: int, row: int) -> float:
 	return _data.elevations_m[_data.index_of(_clamp_column(column), _clamp_row(row))]