Adds a thread-local `HashMap<String, u16>` cache inside
`compute_name_signature`. Repeated calls with the same name (the hot
path inside `MxReferenceHandle::from_names`) skip the `to_lowercase`
allocation and the CRC-16/IBM walk entirely. Bounded at 1024 entries
per thread; on overflow the cache is cleared rather than evicted LRU
— any sane workload re-fills only the names it actively uses.
`MxReferenceHandle::from_names` drops from 2 → 0 allocs/op once warm
(bench delta in `design/M6-bench-baseline.md` § F52.2). Cold-path
behaviour is unchanged: first call with a new name still pays the
`to_lowercase` + cache-key `String` allocations.
Two new tests pin the cache: cache-hit returns the same value as
cold-compute, and cache overflow doesn't break correctness.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Joseph Doherty
a0fa5bedfd