perf: mark error path functions as #[cold] for better optimization

[Boshen]

Summary

This PR adds #[cold] attributes to error handling functions to improve performance by helping the compiler optimize for the common (non-error) path.

Changes

• Added #[cold] attributes to error conversion functions in src/error.rs
• Extracted error creation into separate cold functions in:
  • src/file_system.rs - UTF-8 validation error
  • src/specifier.rs - Empty specifier errors
  • src/windows/mod.rs - Unsupported path error

Benefits

The #[cold] attribute provides hints to the compiler that these functions are unlikely to be called, which enables:

• Better code layout for CPU instruction cache efficiency
• Reduced inline pressure in hot paths
• Improved branch prediction
• Overall better optimization of the happy path

Test Plan

• All existing tests pass
• cargo check passes
• cargo clippy shows no warnings

🤖 Generated with Claude Code

[graphite-app]

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[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.64%. Comparing base (bc9c219) to head (99889c0).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/file_system.rs	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #729      +/-   ##
==========================================
+ Coverage   94.63%   94.64%   +0.01%     
==========================================
  Files          15       15              
  Lines        2908     2914       +6     
==========================================
+ Hits         2752     2758       +6     
  Misses        156      156              

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[codspeed-hq]

CodSpeed Performance Report

Merging #729 will not alter performance

_{Comparing perf-cold-error-paths (99889c0) with main (e2e2d65)¹}

Summary

✅ 7 untouched
⏩ 2 skipped²

Footnotes

1. No successful run was found on main (449e419) during the generation of this report, so e2e2d65 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

2. 2 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. ↩