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fix(builtin,hashmap): let get_or_init/update callbacks write to the map

Map::get_or_init, Map::update, Map::update_or_default and their
@hashmap.HashMap counterparts probed for the key's slot and then called the
user callback, reusing the slot afterwards. A callback that writes to the
same map (memoization is the common case) could leave that slot stale, so
the new entry was written over a live one, behind an emptied slot, or with
a pre-grow mask; `update` could shift back an unrelated slot; and a value
returned for a key the callback removed was written into a detached entry.

- Miss path (`get_or_init`, `update` with `f(None)`): call the callback
  first, then insert with a fresh probe (`set_with_hash`). The extra probe
  is paid on the miss path only.
- Hit path (`update`, `update_or_default`): after `f` returns, the entry is
  used in place only if the (re-masked) slot still holds that same entry
  object; otherwise the result is applied with `set_with_hash` /
  `remove_with_hash`. Comparing sizes would not do: a remove plus an insert
  keeps the size.

The callback's result now wins: it is stored under `key`, replacing any
value the callback stored there itself, and `None` from `update` removes a
key that was present when `f` was called. Doc comments say so.

Signed-off-by: Codex CLI <codex@openai.com>
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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