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Make watcher registration thread safe and retire stale watch bits (#1583)

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## What do these changes do?

`MultiDict_AddWatcher()` and `MultiDict_ClearWatcher()` wrote the
module-wide slot table with plain stores, while event delivery on
another thread read it with no lock. On the free-threaded build that is
a data race, and TSan reports it at `watch.h` against both functions.
Registration now serializes on a `PyMutex` and publishes through new
`freethreading.h` accessors, following CPython's `PyDict_AddWatcher()`.
Delivery stays lock-free.

`ClearWatcher()` also bumps a per-slot generation, and each watch
records the generation it was attached under. A multidict left watched
through a cleared ID used to report to the next registration under that
ID, passing the old registration's `user_data`. That bit is now ignored
until the multidict is watched again. CPython's dict watchers still have
the same reuse hazard; the heap-allocated per-multidict record makes it
cheap to close here. This only affects the C extension: watchers are a C
API feature and have no pure-Python counterpart.

## Are there changes in behavior for the user?

Yes, for C API clients only. Both registration functions are now safe to
call from any thread, and a cleared watcher's leftover watches no longer
reach a later registration that reuses its ID. The docs drop the warning
about that reuse.

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

No. The mutation hot path is unchanged: an unwatched multidict still
pays one pointer test per mutation.

## Related issue number

Follow-up to #1551 and #1574.

## Checklist

- [x] I think the code is well written
- [x] Unit tests for the changes exist
- [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 technic... (continued)

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