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refactor: build to_json result without mutation; lean on check-mode coercion

Apply the comprehension-based build pattern across more ToJson impls,
mirroring the recently-merged Map/SortedMap collapses (#3522 / #3546).
Per maintainer feedback: rely on check-mode coercion — no need to wrap
in `Json::array(...)` when returning `Json`, no need to call
`x.to_json()` inside the comprehension when `X : ToJson` is in scope
(see existing `immut/vector/vector.mbt:669` for the canonical form).

- hashmap/json.mbt — `HashMap::to_json`:
  collapse mutating-Map-build to `Json::object(Map(capacity=..., [for k, v in self => (...)]))`
  (Map shape kept; check-mode unwrap is for arrays only).
- hashset/hashset.mbt — `HashSet::to_json`:
  collapse to bare `[for entry in self.entries if entry is Some({key, ..}) => key]`.
- priority_queue/priority_queue.mbt — drop `Json::array(...)` wrap and inner `to_json()`.
- immut/sorted_set/immutable_set.mbt — same.
- immut/priority_queue/types.mbt — same.
- priority_queue/moon.pkg — move `moonbitlang/core/json` to the test-only
  import block (only README.mbt.md doctest needs `@json.json_inspect`
  now that the impl no longer mentions `Json::`).

Style: labeled args before positional (`capacity=...`).

Semantics-preserving: full `moon test` (6425/6425) passes and
`moon info` produces no `.mbti` change.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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