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Reserve room for bulk adds by free room, not table size (#1617)

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## What do these changes do?

Before a bulk `extend()`, `update()` or `merge()`, the C extension
reserved room by comparing the estimated table size for `used + extra`
with the current size. That ignores the room deleted entries hold, so a
table short of room only because of deletions skipped the reservation
and the adds grew it to three times the live entries mid-call. The
reservation now checks the free room (`usable >= extra`) and otherwise
rebuilds at the size estimated from live entries, which compacts such a
table and can shrink it. This also makes the reservation an actual
guarantee that the adds which follow do not resize.

The pure-Python `update()` and `merge()` used the same size comparison,
and `extend()` resized unconditionally, even when the table already had
room. All three go through one `_reserve()` with the C rule. Its cap at
17 in `update()` and `merge()` is dropped: C never had it, the
pure-Python size is always a real `len()`, and a capped rebuild could
pick a table too small for the live entries.

## Are there changes in behavior for the user?

Bulk adds into a table with many deleted entries compact it instead of
growing it, on both backends.

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

No; both backends now share one rule, and the pure-Python code loses two
branches.

## Related issue number

N/A

## Checklist

- [x] I think the code is well written
- [x] Unit tests for the changes exist
- [ ] Documentation reflects the changes: N/A
- [ ] 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 Opus 5.5 (Claude Code); reviewed by @asvetlov.

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