feat(diff): color file headers

[RestartFU]

Summary

• render Git diff file headers in orange so file boundaries stand out
• paint each file-header row black across the full diff width
• keep addition, deletion, and hunk colors distinct
• assert file-header text and row colors in unified diff tests

Testing

• ./scripts/test.sh (20/20 suites pass)

Closes #115

[coderabbitai]

📝 Walkthrough

Walkthrough

The unified-diff file-title markup now applies a bold #ffbe6f foreground color. The corresponding test verifies that the generated markup styles src/a.c with the expected span.

Changes

Unified-diff title styling

Layer / File(s)	Summary
Title color and markup validation src/util/unified-diff.c, tests/test-unified-diff.c	File titles retain bold styling and now use #ffbe6f; the test asserts the expected markup for src/a.c.

Estimated code review effort: 1 (Trivial) | ~2 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The PR adds a colored foreground to git diff file headers, matching issue #115's request to differentiate files in the diff panel.
Out of Scope Changes check	✅ Passed	The changes are limited to diff header coloring and a matching test, with no unrelated code paths introduced.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the main change: adding color to diff file headers.

✨ 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 xd/https-github-com-restartfu-xd-issues-115-0f82250c

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