Use generation sentinel for transform cache validity and add tests to prevent stale-cache leaks

[TiernanDeFranco]

Motivation

• Remove separate "valid" byte vectors and rely on the node generation field to determine whether a cached global transform is valid, preventing stale cache state when slots are reused.

Description

• Removed global_transform_2d_valid and global_transform_3d_valid fields from TransformRuntimeState and their initialization in new.
• Changed the initial resize value of global_transform_*_generation to use u32::MAX as a sentinel instead of 0 in ensure_global_2d_capacity and ensure_global_3d_capacity.
• Simplified cache-validity checks to compare global_transform_*_generation against id.generation() and removed writes of the removed valid flags when recomputing globals in get_global_transform_2d and get_global_transform_3d.
• Added unit tests in rt_ctx_nodes_transform_api_tests.rs to ensure removed-node cache entries do not leak into reused slot generations and that recomputations overwrite old cached transforms for both 2D and 3D.

Testing

• Ran the unit test suite including the new tests in rt_ctx_nodes_transform_api_tests.rs via cargo test; the tests for reused-slot cache behavior and recompute behavior passed.

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Codex Task