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feat(builtin): add Map::filter

A functional counterpart to Map::retain: returns a new map with the
entries for which the predicate is true, in insertion order. It reuses
each entry's stored hash, sizes the result like the source (no grow),
and skips key comparisons since source keys are unique, so it needs no
`K : Eq` bound. The predicate may raise, like Array::filter.

On 50,000 entries (native, release) it is 2.4x faster than rebuilding
from a comprehension with Int keys and 2.9-4.1x with String keys.

Also clarify Map::retain's docstring: `true` means keep, point to
filter, and name the `f` parameter correctly.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

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