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.