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.