std.fs.path: fix resolveWindows panic with relative drive paths

[Cooksey99]

Summary

In response to Issue #25703

Fixes a panic in std.fs.path.resolveWindows when processing relative drive paths.

Issue

When calling resolveWindows(&[_][]const u8{ "c:foo", "bar" }), the function would panic with "index out of bounds: index 0, len 0".

Root Causes

1. Index out of bounds: compareDiskDesignators tried to access disk_designator[0] without checking if the string was empty
2. Missing disk designator: Relative drive paths didn't write the drive letter to output (produced foo\bar instead of C:foo\bar)
3. Incorrect separator logic: Added \ after drive letter for relative paths (would produce C:\foo instead of C:foo)

Testing

zig build test-std -Dtest-filter="resolveWindows"

[alexrp]

Needs a rebase due to merge conflicts.

[squeek502]

Thanks for working on this!

I'm in the middle of a larger reworking of the Windows std.fs.path stuff, so I'll revisit this when I'm further along and see how it fits in.