feat(fmt): normalize formatter config

[chenjiahan]

Summary

This PR introduces an internal normalized formatter config model so later rs fmt stages can consume project context, shared Prettier options, per-file overrides, and ignore patterns separately.

It adds a pure config transformation without I/O, config factory execution, or public exports.

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 749d537c-ff80-430d-8765-d1c04a7e7140

📥 Commits

Reviewing files that changed from the base of the PR and between ef1daec and 2555374.

📒 Files selected for processing (2)

• packages/rstack/src/fmt/config.ts
• packages/rstack/src/fmt/types.ts

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📝 Walkthrough

Walkthrough

Adds the exported ResolvedFmtConfig interface with Prettier options, root path, overrides, and ignore patterns. Adds and exports normalizeFmtConfig, which converts an optional FmtConfig into this resolved shape and defaults missing overrides and ignore patterns to empty arrays.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: normalizing formatter config.
Description check	✅ Passed	The description is on-topic and describes the normalized formatter config model for rs fmt.
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-normalize-fmt-config

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