path: skip normalize for already-normalized paths

[anonrig]

If a posix path contains no '.' and no '//', it has no '.'/'..' segments and no empty segments, so path.posix.normalize() can return the input without running normalizeString().

This change was implemented with assistance from a closed-source coding agent. I verified the source, official benchmark/path/normalize-posix.js results, and test-path-normalize.js.

Official benches

Three-run medians, n=1e5:

path	main	this PR	delta
/foo/bar	8.4M	13.9M	+66%
/foo	11.6M	15.2M	+31%
/foo/bar//baz/asdf/quux/..	3.13M	2.95M	−6%

Already-normalized paths (the common case) skip the segment walk. Paths that still need normalizeString() pay two indexOf probes.

Tests

test-path-normalize.js, test-path.js, test-path-join.js, test-path-resolve.js.

[nodejs-github-bot]

Review requested:

• @nodejs/path

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.28%. Comparing base (dd5dfb5) to head (f969fe6).
⚠️ Report is 41 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #66177      +/-   ##
==========================================
+ Coverage   90.27%   90.28%   +0.01%     
==========================================
  Files         790      790              
  Lines      271651   272049     +398     
  Branches    51842    51949     +107     
==========================================
+ Hits       245228   245618     +390     
- Misses      16928    16934       +6     
- Partials     9495     9497       +2     

Files with missing lines	Coverage Δ
lib/path.js	97.49% <100.00%> (+<0.01%)	⬆️

... and 61 files with indirect coverage changes

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