moonbitlang / core / 7856

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fix(builtin): keep Map::map memory-safe without snapshotting

A callback that modifies the map only needs to leave the result
memory-safe, not to see a snapshot, so drop the entry/slot snapshot
(+22% on native) and walk the live list again, keeping the result
well-formed instead: link each entry to the one written before it,
derive psl from its slot, skip a slot that is out of range or already
taken, write at most `size` entries, and count what was written.

Map::filter goes back to main's version: it builds its result by
ordinary insertion, so a mutating predicate cannot break it.

The tests now check the result's structure in a bounded whitebox walk,
and keep checking it through set, remove and retain.

Co-Authored-By: Claude <noreply@anthropic.com>

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