Identity stream benchmarks

IdentityTransformStream and FixedLengthStream, under both implementations. Suite rules: src/bench/streams/AGENTS.md.

The byte shapes are 256 x 64 B, 64 x 4 KiB and 16 x 64 KiB (SHAPES in helpers.js). Writes copy their bytes, so each size's chunk is one shared buffer.

Module map

Comparing

• The implementations split and merge writes into reads differently, so the cases count bytes, not reads. Over several queued writes, a BYOB read takes one write's bytes under C++ (stopping once a minimum is met), while the TypeScript implementation fills the view with everything already written (identity test ledger #22). The BYOB reader and readAtLeast cases therefore do fewer reads under TypeScript for the small shapes.
• A write settles once its bytes have been read, in both implementations, so every case reads concurrently with its writes.