fix(perf): replace string-width with fast-string-width

[43081j]

We can use fast-string-width instead which is much faster and smaller, already in wide use too.

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📝 Walkthrough

Walkthrough

This pull request replaces the string-width dependency with fast-string-width in the build package and updates the header renderer’s import. The existing header width computation remains unchanged.

Estimated code review effort: 1 (Trivial) | ~3 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
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.
Title check	✅ Passed	The title clearly matches the main change: replacing string-width with fast-string-width for performance.
Description check	✅ Passed	The description covers the motivation and checklist items, but it omits the required issue reference in the Summary section.

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e18e dependency analysis

No dependency warnings found.

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commit: 824eaf5