Add shared material cache with reference counting

[tracygardner]

Summary

• add a normalized descriptor cache for materials with reference counting and shared metadata
• reuse cached materials for map gradients and default material creation while exposing helpers through the flock reference
• release cached references during mesh disposal and when cloning materials to avoid leaks

Testing

• npm test -- --runTestsByPath tests/materials.test.js (fails: Missing script "test")

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Codex Task