fix: use GLTFDocument to load GLB at runtime (no .import needed)

Godot headless/CLI can't import GLB files — the GLTF loader only works
in the editor GUI. Switched SkinnedPlayerModel to use GLTFDocument.append_from_buffer()
which parses GLB data from raw bytes at runtime.

Verified: GLB parses successfully, generates scene with MikuRig skeleton,
AnimationPlayer, and 7 animations (Crouch, Death, Idle, Idle_001, Jump, Run, Walk).

Also added characters/glb_loader.gd as a reusable utility for loading
any GLB file at runtime.

debug/test_level_builder.gd

@@ -436,6 +436,7 @@ func _spawn_player(pid: int) -> CharacterBody3D:
 		skinned.scale_factor = 1.0  # Already scaled to 1.8m in Blender
 		skinned.position = Vector3.ZERO  # Model origin = player origin
 		player.add_child(skinned)
+		print("TestLevelBuilder: added SkinnedModel for player 1")
 	else:
 		# Default: procedural humanoid
 		var humanoid = load("res://characters/humanoid_model.gd").new()