feat(fmt): add serial runner

[chenjiahan]

Summary

This PR adds the serial execution primitive for the future rs fmt pipeline. It accepts resolved per-file Prettier options, supports write, check, and list-different modes with structured results, and continues after per-file failures with stable exit-code precedence. Writes use bundled atomically, while the minimal code derived from @prettier/cli includes the required license and third-party notices; cache, workers, CLI wiring, and public exports remain out of scope.

Related Links

• https://github.com/prettier/prettier-cli/tree/0a66ef56a70a9bd29a44f3400f5cc7fbfa875341

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

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📥 Commits

Reviewing files that changed from the base of the PR and between fba323c and 13e758a.

⛔ Files ignored due to path filters (1)

• pnpm-lock.yaml is excluded by !**/pnpm-lock.yaml

📒 Files selected for processing (9)

• packages/rstack/THIRD_PARTY_NOTICES.md
• packages/rstack/package.json
• packages/rstack/src/fmt/runner.ts
• packages/rstack/src/fmt/types.ts
• packages/rstack/src/fmt/vendor/prettier-cli/LICENSE
• packages/rstack/src/fmt/vendor/prettier-cli/serial.ts
• packages/rstack/tests/fmt/runner.test.ts
• packages/rstack/tests/fmt/runnerWriteFailure.test.ts
• pnpm-workspace.yaml

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📝 Walkthrough

Walkthrough

Added public formatter execution types and a serial formatter that reads, formats, compares, and optionally writes files atomically. Added runFmtFiles with per-file results, timing, error handling, and aggregate exit codes. Added tests for write, check, list-different, permissions, processing errors, and write failures. Added dependency metadata and third-party license notices.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the main change: adding a serial formatter runner.
Description check	✅ Passed	The description directly explains the serial formatter runner, supported modes, error handling, atomic writes, and scope.
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/feat-fmt-serial-runner

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