Fix assertion during lexical path normalization

[aryansk]

Fixes #283

What changed

• Avoid reparsing mutable compacted storage while asserting the parent-directory invariant during lexical normalization.
• Add a regression case for foo/../../../bar.

Validation

• swift test --filter FilePathSyntaxTest — 6 tests passed.

No documentation or release-note changes are required.

[aryansk]

Thanks @merosm for the approvals on this change — I appreciate the review and the confirmation that the Swift System updates are ready for maintainer handling.

[aryansk]

Thanks again @merosm — I appreciate the additional approval and the continued review of this change. The PR remains available for maintainer handling.