fix(client): address PR review — invalidation, constants, tests

Review feedback from Hoshe + Tyre on PR #8:

- Extract shared TILE_SIZE to Constants class_name (Tyre #5, Hoshe #7)
- Fix tile invalidation: detect tile/visibility count changes instead
  of one-shot dirty flag, supports chunk loading (Tyre #3, #4)
- Fog dirty tracking: only re-render when visible_positions changes
- Add bounds warning for unknown tile types (Hoshe #1)
- Add player-not-found warning in GameState (Hoshe #5)
- Use Image.fill_rect() instead of pixel loops (Tyre #10)
- Document _player_pos as reserved for fog decay #113 (Tyre #6)
- Add TODO(#130) for D-033 relationship colors (Hoshe #3, Tyre #8)
- Add class_name to EntityRenderer, FogRenderer, TileRenderer
- 20 new rendering tests (D-030 Layer 1): entity lifecycle, fog
  registration, tile type mapping, snapshot completeness, constants
- 65 total tests passing, 0 failures

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

client/scripts/autoloads/game_state.gd

@@ -23,10 +23,15 @@ func apply_snapshot(snapshot: Dictionary) -> void:
 	if snapshot.has("entities"):
 		visible_entities = snapshot.entities
 		# Derive player position from the player entity
+		var found_player := false
 		for entity in visible_entities:
 			if entity.has("entity_id") and entity.entity_id == player_entity_id:
 				player_position = Vector2(entity.x, entity.y)
+				found_player = true
 				break
+		if not found_player and visible_entities.size() > 0:
+			push_warning("GameState: player entity_id %d not found in %d entities" % [
+				player_entity_id, visible_entities.size()])
 
 	if snapshot.has("tiles"):
 		visible_tiles = snapshot.tiles