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feat(hashmap): add FromJson impl for HashMap[String, V] to match Map (#3584)

`HashMap` could be serialized to JSON but not read back: `ToJson` has
been there for a while and `FromJson` never landed, so a map could make
a round trip in one direction only. `Map`, `SortedMap` and
`@immut/sorted_map` all carry the object-decoding impl already; this
brings `HashMap` in line with them.

The impl follows `@sorted_map`'s exactly -- an object is required, each
member is decoded with `V`'s impl, and the path is extended with the
member's key so a failure names the entry rather than the whole object.
Since the object's size is known up front, the table is sized once
instead of rehashing as the entries go in.

Restricting the key to `String` matches every other map impl in core.
`ToJson` accepts any `K : Show`, so a round trip is only guaranteed for
`String` keys, which is the same asymmetry `Map` has.

Tests cover a generated round trip, an empty object, nested values, a
non-object input, and the decode path reported when one member fails --
replacing `path.add_key(k)` with `path` fails that last test.

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

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