Native path.win32.relative: use Unicode lowercase comparison

Parent #66; related #898.

Native source c2e235434 is on main through 6417fa68f. Home's private native BunString bridge calls JavaScriptCore's locale-independent Unicode lowercase converter. Native relative paths compare fully lowered strings, use Node's component algorithm when lowercase expands code-unit length, and preserve original destination spelling. Resolve/device comparisons share this comparison. ASCII comparisons retain their direct path, POSIX remains case-sensitive, unpaired surrogates survive, and SDK class/global layouts remain unchanged.

Verified evidence: final native build 56/56; all 464 new actual Node v26.10.0 comparisons pass, including Latin/Greek/Cyrillic/Turkish/contextual sigma/sharp S/supplementary Deseret, UNC shares, drive roots, original spelling, surrogates, collection and long expansion. All earlier 2,601 code-unit comparisons pass. A fresh unchanged 157-file batch has 396 passing cases, 142 process checks, zero failures, 37 case skips and three skipped files; all validators pass. Dirent control, ABI 10 tests/58 assertions, Pickier and the 12,996-entry integrity audit pass.

The initial full ABI/bundler run has 154 passes and one exact-source assertion failure caused by the new private helper. Its assertion now checks both complete source bodies; the repaired case passes with 636 assertions. The initial failure and separately filtered rerun are retained. Installed Bun 1.4.2 still fails the Unicode comparison control and is not a rebuilt pinned reference.

Remaining acceptance:

• Actual Windows execution and drive-specific environment behavior; Darwin path.win32 controls are separate host evidence.
• Cross-platform checks with each linked SDK and a rebuilt pinned-source reference comparison, retained alongside the installed baseline.
• The broader #66 entire feature/suite/all-platform objective remains open. Skips, filtered tests and unsupported hosts are not passing features.

Native path evidence retains the probe source plus actual Node, Home and separately installed Bun outputs for all six Unicode casing mismatches. The native surrogate-preservation batch is published independently on main and does not claim a repair for these results.

Native Unicode path checkpoint: source c2e235434 and 836 Git-verified evidence artifacts are on main through 6417fa68f. Native Windows comparisons use the engine's full locale-independent lowercase conversion and Node's component algorithm when lowercase changes length, preserving destination spelling and POSIX case sensitivity. Build 56/56; 464 new Node comparisons and all 2,601 code-unit comparisons pass. The fresh unchanged 157-file path/filesystem/FileHandle batch has 396 passing cases, 142 process checks, zero failures, 37 case skips and three skipped files; all validators pass. Dirent control, the 12,996-entry corpus audit, ABI checks and lint pass. The full guard run has 154 passes and one repaired source-provenance assertion; the repaired case passes with 636 assertions. Installed Bun 1.4.2 fails the Unicode control and remains a separate baseline. #899 retains actual Windows/platform acceptance; #898 retains drive-environment and rebuilt pinned-reference acceptance. #66 remains the entire feature/suite/all-platform objective.