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Cap the room reserved for a __length_hint__ (#1767)

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## What do these changes do?

`extend()`, `update()` and `merge()` in the C extension reserved room
for whatever `__length_hint__()` reported, so a wild hint such as
`2**40` asked for a huge table and raised `MemoryError`. The hint is now
clamped to `USABLE_FRACTION(2**17)`, the largest table `md_init()`
presizes for; the 17 moved into a shared `HT_LOG_MAX_PRESIZE` constant.
The cap applies only to the early reservation: items past it grow the
table as any add does. Exact tuple, list, dict and multidict sizes are
real counts and stay unclamped.

With the hint bounded, the overflow guards added in #1577 only for such
hints (the saturating keyword addition, the early return in
`md_reserve()` and its twin in `md_reserve_batch()`) are dead and were
removed; `htkeys_size_fits()` stays. The pure-Python backend does no
reservation and is unchanged.

## Are there changes in behavior for the user?

Yes: a huge `__length_hint__()` no longer raises `MemoryError`; the
update completes.

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

No. One `Py_MIN()` on the argument parsing path, and less code on the
reserve path.

## Related issue number

N/A

## Checklist

- [x] I think the code is well written
- [x] Unit tests for the changes exist
- [x] Documentation reflects the changes (N/A, no documented behaviour
changed)
- [ ] 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); pending review by @asvetlov.

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

`test_huge_length_hint_is_capped` replaces
`test_unallocatable_length_hint_raises_memory_error`; its `2**40` case
raises `MemoryError` on master.
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