[F52.2] mxaccess-codec: thread-local name-signature cache

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>

design/M6-bench-baseline.md

@@ -23,7 +23,7 @@ The bench gates on this: any `write_message::encode` scenario at
 | `write_message::encode` (Boolean)              |  10,000 |      1.00 |       37 |        1.00 |
 | `write_message::encode` (String, 5 chars)      |  10,000 |      4.00 |       92 |        4.00 |
 | `write_message::encode_to_bytes_mut` (Int32)   |  10,000 |      2.00 |       44 |        2.00 |
-| `MxReferenceHandle::from_names`                |  10,000 |      2.00 |       22 |        2.00 |
+| `MxReferenceHandle::from_names` (F52.2)        |  10,000 |      0.00 |        0 |        0.00 |
 | `NmxSubscriptionMessage::parse_inner`          |  10,000 |      1.00 |       72 |        1.00 |
 | (DataUpdate, Int32)                            |         |           |          |             |
 
@@ -82,6 +82,23 @@ to fill a pre-sized `&mut [u8]` rather than each allocating their own
 `*_body_size` helpers and resizes the destination buffer (Vec or
 BytesMut) once. This is also the prerequisite refactor for F52.3.
 
+### F52.2 — Per-handle name-signature cache
+
+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` when handles are
+constructed many times) skip the `to_lowercase` allocation entirely.
+Capped at 1024 entries; on overflow the thread's cache is cleared.
+
+| scenario                          | before (allocs/op) | after (allocs/op) |
+|-----------------------------------|-------------------:|------------------:|
+| `MxReferenceHandle::from_names`   |               2.00 |              0.00 |
+
+Cold-path (first call with a new name) still pays the
+`to_lowercase` + cache-key `String` allocations — the cache only helps
+on repeats. The 1k-iter warmup in the F38 harness is enough to prime
+the cache, so the measurement loop sees pure cache hits.
+
 ## Reproducing
 
 ```powershell

design/followups.md

@@ -65,7 +65,7 @@ Array tags (`TestIntArray`, `TestBoolArray`, etc.) read live as `type_id=0 lengt
 
 **Scope.** Three independent codec tightenings, each measurable via the F38 bench harness:
 1. **`bytes::BytesMut` output buffer** on the encoder side. Doesn't reduce alloc count but enables downstream zero-copy splits when the consumer wants to send the encoded body without copying. ✅ Landed 2026-05-06 — `write_message::encode_to_bytes_mut` (and `encode_timestamped_to_bytes_mut`); body builders refactored to fill a pre-sized `&mut [u8]`. Bench delta in `design/M6-bench-baseline.md` § F52.1.
-2. **Per-handle name-signature cache** in `MxReferenceHandle::from_names`. Currently allocates twice (one UTF-16LE conversion per `compute_name_signature` call); cache by `(name, hasher_state)` to elide both on repeated calls with the same names.
+2. **Per-handle name-signature cache** in `MxReferenceHandle::from_names`. Currently allocates twice (one UTF-16LE conversion per `compute_name_signature` call); cache by `(name, hasher_state)` to elide both on repeated calls with the same names. ✅ Landed 2026-05-06 — thread-local `HashMap<String, u16>` keyed by raw name; bounded at 1024 entries. `MxReferenceHandle::from_names` drops 2 → 0 allocs/op once warm. Bench delta in `design/M6-bench-baseline.md` § F52.2.
 3. **Session-level scratch pool** for the per-write encode buffer. Drops the per-write count from 2 → 1 by amortising the output buffer allocation across a session's writes.
 
 **Definition of done:**