refactor: apply for-comprehension to more array-building loops

[bobzhang]

Summary

Third in the series (after #3444 and #3445). Converts five more imperative push-loops to for-comprehensions:

• `argparse/parser.mbt`: `group_usage_expr` (uses `is Some(idx)` binding)
• `argparse/parser_lookup.mbt`: `split_long`
• `env/env_native.mbt`: `get_cli_args_internal`
• `hashset/hashset.mbt`: `ToJson` impl (uses `is Some({ key, .. })` binding)
• `set/linked_hash_set.mbt`: `ToJson` impl

Skipped sites:

• `HashSet::to_array` — hot-path, keeps explicit `capacity=self.size` hint.
• `parser_globals_merge` two-loop concat — needs spread/extend, not a clean comprehension.
• `positional_usage`, `option_entries`, `arg_display` — branching pushes; not idiomatic to unify.

Test plan

• `moon check` clean
• `moon test -p argparse -p env -p hashset -p set` passes (700 tests)
• `moon fmt --check` clean
• `codex review --uncommitted` confirms behavior-preserving across wasm/wasm-gc/js/native targets

🤖 Generated with Claude Code

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 2 additional findings.

[coveralls]

Coverage Report for CI Build 3875

Coverage increased (+0.004%) to 94.918%

Details

• Coverage increased (+0.004%) from the base build.
• Patch coverage: 6 of 6 lines across 4 files are fully covered (100%).
• No coverage regressions found.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	15545
Covered Lines:	14755
Line Coverage:	94.92%
Coverage Strength:	220041.8 hits per line

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💛 - Coveralls

[bobzhang]

Superseded by #3448 which consolidates all comprehension refactors.