fix: create proper Idle animation in Godot when GLB export loses tracks

The Blender GLTF exporter was stripping animation tracks (only 1 of 51
channels survived export). Now SkinnedPlayerModel detects this at
runtime and creates a proper Idle animation programmatically using Godot's
Animation API with 5 bone tracks (Spine, Arms, Neck, Hips).

Also switched model_path back to miku_rigged_final.glb since the
re-export wasn't adding usable animations.

debug/test_level_builder.gd

@@ -432,7 +432,7 @@ func _spawn_player(pid: int) -> CharacterBody3D:
 		# Miku: use the rigged GLB model with its own armature
 		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.model_path = "res://assets/characters/skins/miku_rigged_final.glb"
 		skinned.scale_factor = 1.0  # Already scaled to 1.8m in Blender
 		skinned.position = Vector3.ZERO  # Model origin = player origin
 		player.add_child(skinned)