perf(nl): optimize line numbering by using itoa and direct writing #10757

Performance Gate Passed

⚡ 2 improved benchmarks
✅ 282 untouched benchmarks
⏩ 38 skipped benchmarks¹

	Mode	Benchmark	`BASE`	`HEAD`	Efficiency
⚡	Simulation	`nl_large_file[10]`	58.8 ms	26.9 ms	×2.2
⚡	Simulation	`nl_many_lines[100000]`	46.1 ms	20.8 ms	×2.2

_{Comparing CrazyRoka:optimize-nl-formatting (bccfda0) with main (25a43c6)}

38 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩