Add tangent angle encoding to mesh compression

[beicause]

Objective

Depends on #24572. Perhaps a better alternative to #24535

Solution

Add methods to encode tangent to a snorm16 angle in position.w. Unlike #24535, this only requires both position and tangent to be compressed. Normal precision is unaffected. And in the unit test, this appears to have higher precision than #24535.

The method is mentioned in https://zeux.io/2026/04/30/quantizing-tangent-frames/ and https://www.jeremyong.com/graphics/2023/01/09/tangent-spaces-and-diamond-encoding/

Testing

scene_viewer with --mesh-compression, though testing parallax_mapping (or deferred_rendering) makes it more obvious whether the tangent calculation is correct.