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Keep the cold watcher helpers out of line (#1801)

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## What do these changes do?

Marks `_md_watch_event_name()` as `NOINLINE`, so its switch no longer
gets inlined into the cold `_md_watch_report()`, and drops `inline` from
`watchlog_drain()`. GCC already kept `watchlog_drain()` out of line in
both of its cold callers, so that half only makes the source match the
generated code. The failing-callback path in `_md_watch_call()` was
already under `UNLIKELY` since #1791.

Only `_md_watch_report` (124 to 42 bytes) and the new out of line
`_md_watch_event_name` (91 bytes) change in the binary, on both the GIL
and FT builds. Callgrind instruction counts are identical on every
benchmarked operation, since none of them fires a watcher.

## Are there changes in behavior for the user?

No.

## 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, no behaviour change; the
existing watcher tests cover the paths)
- [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>

Tests: `pytest -q` on CPython 3.11.13, C extension: 4265 passed, 180
skipped. Pure Python (`MULTIDICT_NO_EXTENSIONS=1`, `--no-c-extensions`):
1697 passed, 22 skipped.

Inlining: `tools/check_inlining.py` on 3.14.7 and 3.14.7t: ok, 13 rules
and no new CPython inline copies.

Callgrind (`benchmarks/callgrind_driver.py --include-multidict-only`,
3.14.7 GIL and 3.14.7t FT, base `dbf7eeed`): all 31 operations for
`multidict_c` and `cimultidict_c` are equ... (continued)

842 of 1684 branches covered (50.0%)

Branch coverage included in aggregate %.

9799 of 10709 relevant lines covered (91.5%)

1.83 hits per line