Preserve leading .. for drive-relative paths during lexical normalization

[adityasingh2400]

On Windows, a traditional drive-relative root like C: anchors a path to the current working directory on the named drive, not to a fixed location. That makes a leading .. meaningful: C:.. is the parent of the drive's current directory, which is generally not the root of the drive. lexicallyNormalize() was treating any root as a fixed anchor and collapsing leading .. against it, so it silently changed the meaning of drive-relative paths:

FilePath(#"C:..\foo\bar"#).lexicallyNormalized()    // produced C:foo\bar
FilePath(#"C:..\..\foo"#).lexicallyNormalized()     // produced C:foo
FilePath(#"C:foo\..\..\bar"#).lexicallyNormalized() // produced C:bar

In each case a .. that escapes the relative portion was dropped, which loses information.

The root cause is in _normalizeSpecialDirectories: the branch that skips a leading .. fired whenever the path had a root (hasRoot && writeIdx == relStart). That is correct for fixed anchors (/ on Unix, and \, C:\, UNC and device roots on Windows, where .. at the front refers to the parent of the root and collapses to the root), but it is wrong for the drive-relative C: form, where the .. has to be kept just like a leading .. on a rootless relative path.

The fix distinguishes the two cases. A new internal Root._isTraditionalDriveRelative identifies the X: form (a two-character root ending in :, which the existing isAbsolute logic already treats as the only colon-terminated relative root). Normalization now preserves a leading .. exactly when the root, if any, is drive-relative. isLexicallyNormal is updated to match, which resolves the FIXME that was sitting on it: C:..\foo\bar is now reported as lexically normal, while \..\foo\bar is not.

After the change:

FilePath(#"C:..\foo\bar"#).lexicallyNormalized()    // C:..\foo\bar
FilePath(#"C:..\..\foo"#).lexicallyNormalized()     // C:..\..\foo
FilePath(#"C:foo\..\..\bar"#).lexicallyNormalized() // C:..\bar
FilePath(#"\..\foo\bar"#).lexicallyNormalized()    // \foo\bar  (unchanged)

These match ntpath.normpath for the same inputs (CPython recently corrected its own drive-relative normalization for the same reason in python/cpython#126780). The rooted \ and absolute cases, and all Unix behavior such as /.. => /, are unaffected.

Verification: reproduced the wrong output against the documented/reference behavior using the package's withWindowsPaths test harness on macOS, applied the fix, and confirmed the outputs match. Added regression cases in FilePathSyntaxTest covering leading and interior .. for both drive-relative (C:) and rooted (\) Windows paths; they fail before the change and pass after. swift test passes in both debug and release (67 XCTest tests and 7 swift-testing tests, 0 failures), and one pre-existing case that encoded the old behavior (C:foo\bar\..\..\.. => C:) was corrected to C:...

This is related to the drive-root discussion in #188 (the trapping/validation questions there are separate and not addressed here).

I have not yet completed the Swift project CLA; happy to sign it as part of the review process per CONTRIBUTING.