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.

scenes/maps/level_runtime.gd

@@ -148,9 +148,10 @@ func _spawn_player(pid: int) -> CharacterBody3D:
 		var skinned = load("res://characters/skinned_player_model.gd").new()
 		skinned.name = "SkinnedModel"
 		skinned.model_path = "res://assets/characters/skins/miku_rigged_animated.glb"
-		skinned.position = Vector3(0, 0.0, 0)  # Model origin aligns with player origin
-		skinned.first_person_mode = true  # Hide body in FPS view, show only arms
+		skinned.position = Vector3(0, 0.0, 0)
+		skinned.first_person_mode = true
 		player.add_child(skinned)
+		print("LevelRuntime: added SkinnedModel for local player")
 	else:
 		# Remote players: use procedural humanoid
 		var humanoid = load("res://characters/humanoid_model.gd").new()