refactor(client): replace fog byte magic numbers with named constants

Define VIS_HIDDEN/PERIPHERAL/FORWARD and EXP_UNEXPLORED/EXPLORED/
VISIBLE in FogState. Replaces 7 magic number usages in _resize() and
update_from_state(). Provides stable assertion targets for fog tests.
Implements #476.

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

client/scripts/autoloads/fog_state.gd

@@ -4,6 +4,17 @@ extends Node
 ## Read by fog_shader.gd for shader uniforms. Not a renderer — pure data.
 ## Architecture: docs/architecture/fog-shader-spec.md | D-059
 
+# Fog texture byte values — visibility and exploration layers.
+# Used by fog shader to distinguish visual treatment per tile.
+# Test assertions reference these: assert_that(byte).is_equal(FogState.VIS_FORWARD)
+const VIS_HIDDEN: int = 0        # Not in LOS — fully fogged
+const VIS_PERIPHERAL: int = 180  # In LOS, peripheral sector — light fog dimming
+const VIS_FORWARD: int = 255     # In LOS, forward sector — clear vision
+
+const EXP_UNEXPLORED: int = 0   # Never seen — total darkness
+const EXP_EXPLORED: int = 128   # Previously seen, now out of LOS — deep fog
+const EXP_VISIBLE: int = 255    # Currently in LOS — clear (written each frame)
+
 var map_bounds: Rect2i = Rect2i(0, 0, 1, 1)
 var visibility_texture: ImageTexture
 var exploration_texture: ImageTexture
@@ -31,13 +42,13 @@ func _resize(bounds: Rect2i) -> void:
 
 	_vis_bytes = PackedByteArray()
 	_vis_bytes.resize(sz)
-	_vis_bytes.fill(0)
+	_vis_bytes.fill(VIS_HIDDEN)
 	_vis_image = Image.create_from_data(_width, _height, false, Image.FORMAT_R8, _vis_bytes)
 	visibility_texture = ImageTexture.create_from_image(_vis_image)
 
 	_exp_bytes = PackedByteArray()
 	_exp_bytes.resize(sz)
-	_exp_bytes.fill(0)
+	_exp_bytes.fill(EXP_UNEXPLORED)
 	_exp_image = Image.create_from_data(_width, _height, false, Image.FORMAT_R8, _exp_bytes)
 	exploration_texture = ImageTexture.create_from_image(_exp_image)
 
@@ -62,21 +73,21 @@ func update_from_state() -> void:
 	var positions: Dictionary = GameState.visible_positions
 	var sectors: Dictionary = GameState.visibility_sectors
 
-	# TODO(v0.2): gradual decay over game-time instead of immediate 255→128
+	# TODO(v0.2): gradual decay over game-time instead of immediate EXP_VISIBLE→EXP_EXPLORED
 
 	# 1. Clear visibility, then write current LOS
-	_vis_bytes.fill(0)
+	_vis_bytes.fill(VIS_HIDDEN)
 	for pos in 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
 		var sector: String = sectors.get(pos, "Forward")
-		_vis_bytes[py * _width + px] = 255 if sector == "Forward" else 180
+		_vis_bytes[py * _width + px] = VIS_FORWARD if sector == "Forward" else VIS_PERIPHERAL
 	_vis_image.set_data(_width, _height, false, Image.FORMAT_R8, _vis_bytes)
 	visibility_texture.update(_vis_image)
 
-	# 2. Exploration: tiles leaving LOS decay to 128, visible tiles stay 255
+	# 2. Exploration: tiles leaving LOS decay to EXP_EXPLORED, visible tiles stay EXP_VISIBLE
 	#    Only touch tiles that changed (O(visible) not O(map_size))
 	for pos in _prev_visible:
 		if not positions.has(pos):
@@ -84,14 +95,14 @@ func update_from_state() -> void:
 			var py: int = pos.y - oy
 			if px >= 0 and py >= 0 and px < _width and py < _height:
 				var idx: int = py * _width + px
-				if _exp_bytes[idx] > 128:
+				if _exp_bytes[idx] > EXP_EXPLORED:
-					_exp_bytes[idx] = 128
+					_exp_bytes[idx] = EXP_EXPLORED
 
 	for pos in positions:
 		var px: int = pos.x - ox
 		var py: int = pos.y - oy
 		if px >= 0 and py >= 0 and px < _width and py < _height:
-			_exp_bytes[py * _width + px] = 255
+			_exp_bytes[py * _width + px] = EXP_VISIBLE
 	_exp_image.set_data(_width, _height, false, Image.FORMAT_R8, _exp_bytes)
 	exploration_texture.update(_exp_image)