[codex] add fn syntax to trait implementations

[bobzhang]

Summary

Split out the safe fn syntax portion from the broader formatter/info PR.

• add fn to trait member declarations where needed
• update impl ... with declarations to impl ... with fn
• regenerate src/io/pkg.generated.mbti with the matching public trait syntax

This intentionally does not include the moon fmt manifest migration or the broader formatter/test-helper changes from #394.

Validation

• moon check --deny-warn
• moon check --target js --deny-warn
• moon info
• git diff --check

moon test --deny-warn was also run locally, but this environment reported existing-looking fs/process timing and file-lock failures unrelated to this syntax-only change: fs/lock_test.mbt and process/spawn_in_group_test.mbt / process/cancel_test.mbt.

[coveralls]

Coverage Report for CI Build 361

Coverage increased (+0.05%) to 79.332%

Details

• Coverage increased (+0.05%) from the base build.
• Patch coverage: No coverable lines changed in this PR.
• No coverage regressions found.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	4190
Covered Lines:	3324
Line Coverage:	79.33%
Coverage Strength:	80303.71 hits per line

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💛 - Coveralls

[bobzhang]

CI status after monitoring:

• Passing: moon-info, wasm-check, js-build, coverage on Unix/Windows, sanitizer on Unix/Windows, stable builds on macOS/Ubuntu/Windows, nightly builds on macOS/Ubuntu/Windows, misc checks.
• Failing: moon-format only.

I checked the formatter log. The failure is the broader formatter migration already split into #394: moon fmt migrates moon.mod.json to moon.mod and applies unrelated formatting wraps. This PR intentionally keeps only the fn syntax update plus the generated src/io/pkg.generated.mbti change needed for moon-info, so I did not pull those formatter changes into this small PR.