Phase 5 cabin polish — door, floor, interior crate, double-render fix

Make the demo cabin readable as a real building so the rendering pattern
is solid before Phase 6+ adds more building types.

Demo seed (world.gd._seed_phase5_demo_buildings):
- 8×6 stone cabin at (44, 23) — 23 walls (perimeter minus door slot) +
  1 door (south wall centre at (47, 28)) + 24 wood-floor designations
  for the interior. ConstructionProvider picks them all up; pawns build
  the whole thing.
- One pre-built crate inside at (50, 24) so the interior reads as a
  furnished room on first frame.
- Two external stockpile-target crates unchanged at (17, 60) / (18, 60).

Door visual rewrite (door.gd):
- Was the old 16×24 bottom-anchored shape that encroached on the cell
  above. Now fits strictly within its 16×16 tile, matching the wall's
  3/4 band layout (5 px lit lintel + 11 px shaded frame + inset panel
  + hinge dot). Door and walls now share a top horizon line so they
  line up visually.

Designation gained TOOL_BUILD_DOOR + atlas mapping; world.gd's
_on_designation_added now branches on build_door to spawn a Door entity.

THE DOUBLE-RENDER BUG (caught by MCP inspection):
- World.mark_floor_tile stamps the Floor TileMap with atlas (2, 0) which
  is *stone-grey* in the placeholder atlas, regardless of material name.
- The Floor TileMap layer was visible=true with z_index=1, so it drew
  ON TOP of the brown Floor entity sprites underneath.
- Result before fix: interior tiles looked gray-stone, not wood.
- Fix: set Floor TileMap layer visible=false (data-only, same as Wall).
  Entities own the visual; the TileMap retains tile-level data for
  Phase 13's room detection + Phase 16's save format.

Pattern locked for future building types: 'render at entity level,
TileMap layers are data-only'. Phase 13's roof and any future wall
materials follow the same template.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scenes/world/world.tscn

@@ -20,6 +20,7 @@ z_index = 0
 
 [node name="Floor" type="TileMapLayer" parent="."]
 z_index = 1
+visible = false
 
 [node name="Wall" type="TileMapLayer" parent="."]
 z_index = 2