Fix FBX animation refinement growing across bones - #19050

🤖 This PR was created by the create-pr skill.

Summary

Fix the out-of-memory failure when importing animated Mixamo FBX files reported in this forum topic.

_buildInheritedRigBoneAnimations() previously fed each bone's refined sample-time array into the next bone's refinement pass. Although each pass limits recursion to two levels, that limit restarted for each bone, allowing subdivisions to compound across the rig.

Each bone now refines the same original sample intervals. The results are merged and sorted into a shared sampling grid. This preserves authored key times and adaptive refinement while bounding subdivisions to the existing depth limit independently of the number of animated bones.

Normalize the decomposed and interpolated quaternions used for the angular-error comparison. Small length errors, including the near-linear branch of quaternion interpolation, must not be interpreted as angular differences.

No public API or loader options change.

Regression coverage

• A four-bone inherited rig previously generated 1,649 keys per rotation track; the test now requires no more than 121 keys while retaining adaptive samples and the complete clip range.
• A small constant-speed rotation no longer gets extra samples from quaternion length error.
• Constant rotations remain unrefined, and nonlinear translation retains authored times and the existing subdivision depth.

Validation

• All 113 FBX unit tests pass.
• Changed-file Prettier and ESLint checks pass (ESLint reports two pre-existing warnings outside the change).
• A focused TypeScript check of the changed loader and tests passes with the core ambient declarations and generated shader prerequisites.
• Loader-only tree-shaking invariants and side-effects synchronization checks pass.
• The reporter's mixamo.fbx imports with the patched loader against Babylon.js 9.32.0 in a WebGL browser, without disabling refinement: 2 meshes, 1 skeleton, 1 animation group, 49 channels, approximately frames 0–505, and at most 2,499 keys per track. Animation advancement was confirmed after 120 rendered frames.
• Before the fix, instrumented loading of the same asset grew from 999 sample times to 21,422 within eleven bone passes; the diagnostic stopped there to avoid exhausting memory.

Validation is scoped to the affected loader; no full-repository build or test run was performed.