feat(fmt): add single-file formatting core

[chenjiahan]

This PR adds the internal single-file formatting core needed by the future rs fmt pipeline. It resolves per-file options, infers supported parsers without loading external Prettier config or ignore files, and maps cursor positions when requested. Unsupported file types return a structured skip result, while plugin handling remains deferred until project-root plugin resolution is implemented. This change does not add an rs fmt command or public package export.

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[coderabbitai]

📝 Walkthrough

Walkthrough

Adds the typed formatText API and structured formatted or unsupported results. The implementation resolves formatting options, rejects configured plugins, infers parsers from file paths, supports cursor-aware formatting, and formats paths under node_modules. Tests cover per-file overrides, cursor mapping, unknown extensions, explicit parsers, and node_modules paths.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: adding a single-file formatting core.
Description check	✅ Passed	The description matches the PR goals and summarizes the formatting core changes accurately.
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-format-text-core

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[coderabbitai]

🧹 Nitpick comments (1)

packages/rstack/tests/fmt/format.test.ts (1)

1-77: 📐 Maintainability & Code Quality | 🔵 Trivial | ⚡ Quick win

Add regression coverage for plugin rejection.

packages/rstack/src/fmt/format.ts explicitly rejects configured plugins, but this changed test file does not verify that contract. Add a test asserting that formatText rejects with the expected error when plugins are configured.

🤖 Prompt for 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.

In `@packages/rstack/tests/fmt/format.test.ts` around lines 1 - 77, Add a
regression test alongside the existing formatText tests that configures at least
one plugin, invokes formatText with that configuration, and asserts it rejects
with the expected plugin-rejection error from format.ts. Keep the assertion
focused on the established error contract and preserve the existing test
patterns.

🤖 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.

Nitpick comments:
In `@packages/rstack/tests/fmt/format.test.ts`:
- Around line 1-77: Add a regression test alongside the existing formatText
tests that configures at least one plugin, invokes formatText with that
configuration, and asserts it rejects with the expected plugin-rejection error
from format.ts. Keep the assertion focused on the established error contract and
preserve the existing test patterns.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: d3c529e1-a7e8-4a49-b075-c9529a759d0c

📥 Commits

Reviewing files that changed from the base of the PR and between 6c563ef and 8363eb7.

📒 Files selected for processing (3)

• packages/rstack/src/fmt/format.ts
• packages/rstack/src/fmt/types.ts
• packages/rstack/tests/fmt/format.test.ts