refactor(fmt): prepare runner for multiple execution paths

[chenjiahan]

Summary

• extract shared per-file formatting and result conversion
• separate serial scheduling from exit-code aggregation
• preserve the current serial behavior while preparing the runner for another execution path

Testing

• pnpm --filter rstack build
• pnpm --filter rstack test
• pnpm lint
• pnpm check:format

[coderabbitai]

📝 Walkthrough

Walkthrough

The formatting runner now separates per-file execution, sequential file processing, and exit-code calculation. It records per-file duration, maps results to written, different, unchanged, or error states, and preserves input order. Exit-code calculation returns 2 for errors, 1 for differing files, and 0 otherwise. runFmtFiles delegates to the new helpers while retaining write-mode behavior and aggregate timing.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly describes the formatting runner refactor and its preparation for multiple execution paths.
Description check	✅ Passed	The description directly explains the formatting runner refactor and lists relevant validation commands.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch chenjiahan/refactor-fmt-runner-execution-paths

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@packages/rstack/src/fmt/runner.ts`:
- Around line 69-79: Export the runFmtFiles function so it can be invoked by
other modules; preserve its existing parameters, serial execution, result
construction, and timing behavior.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 10ef89d1-eb93-4ee7-8b1d-ef9cca73a6fb

📥 Commits

Reviewing files that changed from the base of the PR and between fefc003 and 9411ada.

📒 Files selected for processing (1)

• packages/rstack/src/fmt/runner.ts

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