FBX animation support

**Source note:** "FBX animations"

### Proposal
Support FBX animation import and playback.

### Related
- #397 (asset viewer for FBX)
- #366 (Unity/Unreal asset import)

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_Filed from handwritten planning notes (2026-07-23). Description expanded for clarity — technical suggestions are marked as such and are not confirmed against the codebase._