build: baked lighting pass on kit_demo test map

Restructured kit_demo.tscn from linear demo to enclosed 5.12x5.12 room:
- 2x2 floor tiles, 8 wall segments (including doorway + window)
- Pillar at room center, beam overhead, accent panels
- West/east walls rotated 90° Y for proper enclosure

Lighting infrastructure:
- WorldEnvironment with ambient light
- DirectionalLight3D (sun key light, 45° angle, shadows enabled)
- OmniLight3D (warm interior fill light)
- ReflectionProbe (box projection, room-sized extents)
- LightmapGI (Quality=High, bounces=3, denoiser=true)
- Tool script prints bake status in editor

Scripts:
- bake_lighting.gd: validates LightmapGI config from CLI
- validate_scene.gd: full scene validation (17 checks, all pass)

Note: Godot 4.7 removed LightmapGI.bake() from runtime API.
Baking must be done in the editor: select LightmapGI node > Bake Lightmap.

client/scripts/validate_scene.gd

@@ -0,0 +1,101 @@
+extends SceneTree
+
+# Validate kit_demo.tscn scene structure.
+# Usage: godot --display-driver x11 --rendering-driver opengl3 --script scripts/validate_scene.gd
+
+func _init():
+	print("=== Validating kit_demo.tscn ===")
+	
+	var scene = ResourceLoader.load("res://assets/scenes/modular/kit_demo.tscn")
+	if not scene:
+		printerr("ERROR: Could not load kit_demo.tscn!")
+		quit(1)
+		return
+	
+	print("Scene loaded successfully.")
+	
+	var instance = scene.instantiate()
+	if not instance:
+		printerr("ERROR: Could not instantiate scene!")
+		quit(1)
+		return
+	
+	root.add_child(instance)
+	print("Scene instantiated successfully.")
+	print("")
+	
+	# Validate key nodes
+	var checks = _check_nodes(instance)
+	
+	# Print results
+	var passed = 0
+	var failed = 0
+	for check in checks:
+		var status = "✓" if check[1] else "✗"
+		if check[1]:
+			passed += 1
+		else:
+			failed += 1
+		print("  ", status, " ", check[0])
+	
+	# General info
+	var node_count = _count_nodes(instance)
+	print("")
+	print("Scene stats:")
+	print("  Total nodes:         ", node_count)
+	print("  Checks passed:       ", passed)
+	print("  Checks failed:       ", failed)
+	print("")
+	
+	if failed > 0:
+		printerr("Validation FAILED — ", failed, " check(s) failed.")
+		quit(1)
+	else:
+		print("Validation PASSED.")
+		quit(0)
+
+
+func _check_nodes(root_node: Node) -> Array:
+	var r = []
+	
+	# Built-in lighting nodes
+	r.append(["WorldEnvironment (Environment+Sky)", root_node.find_child("WorldEnvironment", true, false) != null])
+	r.append(["DirectionalLight3D (SunLight)", root_node.find_child("SunLight", true, false) != null])
+	r.append(["OmniLight3D (FillLight)", root_node.find_child("FillLight", true, false) != null])
+	r.append(["ReflectionProbe", root_node.find_child("ReflectionProbe", true, false) != null])
+	r.append(["LightmapGI", root_node.find_child("LightmapGI", true, false) != null])
+	
+	# Lightmap config
+	var lm = root_node.find_child("LightmapGI", true, false)
+	if lm:
+		r.append(["LightmapGI.bounces = 3", lm.bounces == 3])
+		r.append(["LightmapGI.interior = true", lm.interior == true])
+		r.append(["LightmapGI.use_denoiser = true", lm.use_denoiser == true])
+		r.append(["LightmapGI.texel_scale = 1.0", abs(lm.texel_scale - 1.0) < 0.01])
+	
+	# Module pieces (floor, walls, pillar, beam, accent panels)
+	r.append(["Floor tiles (4 pieces, one ceramic)", _count_instances(root_node, "Floor") >= 4])
+	r.append(["Wall segments (8 total)", _count_instances(root_node, "Wall") + _count_instances(root_node, "South") + _count_instances(root_node, "North") + _count_instances(root_node, "East") + _count_instances(root_node, "West") >= 8])
+	r.append(["Pillar (center)", root_node.find_child("Pillar", true, false) != null])
+	r.append(["Beam (overhead)", root_node.find_child("Beam", true, false) != null])
+	r.append(["AccentRed panel", root_node.find_child("AccentRed", true, false) != null])
+	r.append(["AccentBlue panel", root_node.find_child("AccentBlue", true, false) != null])
+	r.append(["Doorway segment", root_node.find_child("SouthDoorway", true, false) != null])
+	r.append(["Window segment", root_node.find_child("NorthWindow", true, false) != null])
+	
+	return r
+
+
+func _count_nodes(parent: Node) -> int:
+	var count = 1
+	for child in parent.get_children():
+		count += _count_nodes(child)
+	return count
+
+
+func _count_instances(parent: Node, name_prefix: String) -> int:
+	var count = 0
+	for child in parent.get_children():
+		if child.name.begins_with(name_prefix):
+			count += 1
+	return count