fix(ui): render LOS boundary wall tiles through fog without marking explored (#585)

- game_state.gd: add boundary_positions Dictionary field; BoundaryWall tiles from
  visible_tiles go to boundary_positions instead of visible_positions — rendered by
  tile_renderer but not tracked as explored fog memory
- fog_state.gd: update_from_state() writes VIS_FORWARD for boundary_positions so fog
  lifts over margin wall content; boundary tiles excluded from exploration step so they
  don't persist as EXP_EXPLORED when player turns away
- tile_renderer.gd: no changes needed — renders all visible_tiles by type, sector-agnostic
- test_fog_shader.gd: 4 new tests — boundary excluded from visible_positions, tracked in
  boundary_positions, cleared each snapshot, fog lifts to VIS_FORWARD

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

client/scripts/autoloads/fog_state.gd

@@ -161,6 +161,14 @@ func update_from_state() -> void:
 		if px < 0 or py < 0 or px >= _width or py >= _height:
 			continue
 		_vis_bytes[py * _width + px] = VIS_FORWARD
+	# #585: BoundaryWall margin tiles — fog lifts so wall content composites correctly,
+	# but NOT in visible_positions so they don't persist as explored memory.
+	for pos in GameState.boundary_positions:
+		var px: int = pos.x - ox
+		var py: int = pos.y - oy
+		if px < 0 or py < 0 or px >= _width or py >= _height:
+			continue
+		_vis_bytes[py * _width + px] = VIS_FORWARD
 	_vis_image.set_data(_width, _height, false, Image.FORMAT_R8, _vis_bytes)
 	visibility_texture.update(_vis_image)
 

client/scripts/autoloads/game_state.gd

@@ -18,7 +18,8 @@ var current_tick: int = 0
 var player_position: Vector2 = Vector2.ZERO
 var visible_entities: Array = []
 var visible_tiles: Array = []
-var visible_positions: Dictionary = {}  # Vector2i -> true, for fast fog lookups
+var visible_positions: Dictionary = {}   # Vector2i -> true, for fast fog lookups (normal LOS tiles)
+var boundary_positions: Dictionary = {}  # Vector2i -> true, BoundaryWall margin tiles (#585) — visible in fog but not explored
 
 # v2 fields (D-015, D-031)
 var game_time: Dictionary = {}  # {day, time_of_day, day_phase, tick_rate} or empty
@@ -347,17 +348,24 @@ func apply_snapshot(snapshot: Dictionary) -> void:
 		current_zone_id = player_tile.get("zone_id", "") if player_tile else ""
 
 	# v2: visible_tiles with visibility sectors
-	# Derives visible_positions when not explicitly provided (real server mode)
+	# Derives visible_positions when not explicitly provided (real server mode).
+	# #585: BoundaryWall tiles go to boundary_positions — rendered in fog but not marked explored.
 	if snapshot.has("visible_tiles") and snapshot.visible_tiles is Array and snapshot.visible_tiles.size() > 0:
 		visibility_sectors.clear()
 		var has_explicit_positions := snapshot.has("visible_positions")
 		if not has_explicit_positions:
 			visible_positions.clear()
+		boundary_positions.clear()
 		for vtile in snapshot.visible_tiles:
 			if not vtile is Dictionary or not vtile.has("x") or not vtile.has("y"):
 				continue
 			var pos := Vector2i(vtile.x, vtile.y)
+			var vis_sector: String = vtile.get("visibility", "")
 			if vtile.has("visibility"):
-				visibility_sectors[pos] = vtile.visibility
-			if not has_explicit_positions:
+				visibility_sectors[pos] = vis_sector
+			# #585: BoundaryWall tiles are margin tiles visible through fog but not persistently
+			# explored — they don't update the player's exploration memory when they leave LOS.
+			if vis_sector == "BoundaryWall":
+				boundary_positions[pos] = true
+			elif not has_explicit_positions:
 				visible_positions[pos] = true