refactor - MMD Model Loading

[Lilia-Chen]

PR Summary

• Introduced a pluggable assembly pipeline via loader-deps.ts: parse → post-process → geometry → materials → bones → IK → grants → mesh → physics are all overridable through a Plugin that returns Partial<ThreeMMDLoaderDeps>.
• Added physics abstraction: PhysicsStrategy + default factory buildPhysics; spring-bone implementation is now a functional factory createSpringBonePhysics (closure-backed state, swap-friendly).
• Clarified Loader/MMD responsibilities: Loader assembles (parse/post-process/build…/physics); MMD holds the assembled mesh/constraints/physics strategy and exposes lifecycle hooks.
• Centralised Z-flip in post-parse to normalise PMX to Three’s handedness; downstream builders should drop duplicate flips.

Design Highlights

• Functional core + OOP shell: pure build funcs produce data, Loader/OOP handles assembly and lifecycle.
• Pluginable deps: register(plugin) overrides specific steps by key; materials/physics/parse strategies can be hot-swapped.
• Physics as a strategy: defaults to spring-bone, but buildPhysics/PhysicsStrategy<THelpers> allows other engines or disabling physics.

https://www.tldraw.com/f/-mkWrojbWfUXMsREqHs33?d=v248.-588.2300.1271.h6bzJK397xylpxp3u8dVm

Next Step

• Refactor buildMaterials to support more flexible and varied materials and shading.
• Refactor build-animation related code