aio-libs / multidict / 36149088889

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Order MultiDictObject fields hot to cold and drop its module reference (#1584)

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## What do these changes do?

Reorders the fields of the C extension's `MultiDictObject` from hot to
cold: the ones a lookup reads (`state`, `keys`, `num_active_readers` on
free-threaded builds, `is_ci`) come right after the object header, the
ones a mutation writes follow, and `retired` and the pre-3.12 `weaklist`
go last.

It also drops `MultiDictObject.mod`. Every multidict already reached the
same module through `state->mod`, so the object now owns its strong
reference to `state->mod` and releases it in dealloc and visits it in
traverse the same way. `state` stays addressable through teardown
exactly as before. `sizeof` drops from 72 to 64 bytes on the GIL build
and from 104 to 96 on the free-threaded one, which moves the allocation
into a smaller size class on both.

This only touches the C extension; the pure-Python implementation has no
equivalent layout.

## Are there changes in behavior for the user?

No. Each multidict uses 16 bytes less memory on 3.12+.

## Is it a substantial burden for the maintainers to support this?

No.

## Related issue number

N/A

## Checklist

- [x] I think the code is well written
- [ ] Unit tests for the changes exist; N/A, layout only, covered by the
existing suite
- [x] Documentation reflects the changes
- [ ] If you provide code modification, please add yourself to
`CONTRIBUTORS.txt`; N/A
- [x] Add a new news fragment into the `CHANGES/` folder
- [x] `make doc-spelling` passes and any new technical words are added
to `docs/spelling_wordlist.txt`

Drafted with Claude Code (Claude Opus 5.5); reviewed by @asvetlov.

<details>
<summary>Agent run details (optional, for reviewers)</summary>

Measured on CPython 3.14.7 and 3.14.7t, x86-64.

Callgrind driver (`--include-multidict-only`), Ir per op, every other
row within 0.5%:

| operation | build | before | after |
|---|---|---|---|
| `ctor_empty... (continued)

780 of 1560 branches covered (50.0%)

Branch coverage included in aggregate %.

8086 of 8624 relevant lines covered (93.76%)

1.88 hits per line