Reduce allocations in Matrix4Tween.lerp to improve transform animation performance - #190659

Fixes #32055

Matrix4Tween.lerp currently allocates decomposition outputs plus temporary translation, rotation, and scale objects on every animation tick. A single interpolation creates at least 16 avoidable vector and quaternion wrappers, along with their typed-data storage, before composing the required result matrix. This adds allocation and garbage-collection pressure to transform animations.

The solution is to retain six lazily initialized scratch objects per tween, decompose the current endpoints into them on every call, and perform the existing scale, add, and normalize operations in place. The arithmetic order and normalized quaternion interpolation remain unchanged, mutable endpoints are still observed on every evaluation, and each call continues to return an independent Matrix4.

Performance

Measured with the maintained matrix4_tween_bench.dart benchmark using Dart 3.14 on macOS arm64. Each reported runtime covers 2,050 lerps and is the median of five runs.

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