Bug-triage patch — fix torch builds, idle-pawn traps, floor render order

Three playtest-reported bugs fixed out-of-phase before Phase 18:

* Furniture build-queue gap: Torch / Bed / Crate / Workbench / CremationPyre
  were missing World.register_build_site(self) in _ready, so newly-painted
  designations never entered ConstructionProvider's iteration. The seeded
  cabin pre-built everything via _spawn_complete_* helpers, masking the gap
  until a player painted a fresh furniture designation.

* Wall-trap regression for bystanders + walk-through pawns: Wall._complete
  now dislodges any pawn on the tile via new Pathfinder.find_nearest_walkable
  BFS helper; Pawn._advance_walk re-checks next tile walkability before
  stepping, aborts walk + cancels job + lets Decision reroute. Phase 6's
  adjacent-stand fix only protected the BUILDING pawn.

* Floor / Pawn Y-sort ambiguity: Floor was anchored at tile-center
  (same Y as Pawn), so Y-sort tiebreak fell to scene-tree order and
  Floor (spawned later) drew over Pawn. Moved Floor origin to top-of-tile
  so Floor.y < Pawn.y under Y-sort; _draw rect offsets compensate.

All three verified via MCP runtime: torch built end-to-end, all 3 pawns
working on different jobs with no idle traps, pawn renders over floor.

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

scenes/entities/torch.gd

@@ -73,6 +73,7 @@ func _ready() -> void:
 		tile.y * TILE_SIZE_PX + TILE_SIZE_PX
 	)
 	World.register_light_source(self)
+	World.register_build_site(self)
 	_light = _build_point_light_2d()
 	add_child(_light)
 	_light.enabled = false   # dark until built
@@ -81,6 +82,7 @@ func _ready() -> void:
 
 func _exit_tree() -> void:
 	World.unregister_light_source(self)
+	World.unregister_build_site(self)
 
 
 ## One-shot initialiser. Call after add_child() so _ready() has already fired.