fix: open Windows drive, UNC and MSYS paths from hints

[abcfy2]

Fixes #1943.

Problem

On Windows, clicking a hinted path that is not a scheme URL silently does nothing for the most common native and MSYS spellings:

• drive-letter paths: C:\Users\me\notes.txt, C:/Users/me/notes.txt
• UNC shares: \\server\share\doc.pdf, //server/share/doc.pdf
• MSYS/Cygwin/WSL roots: /c/Users/me/notes.txt, /cygdrive/c/..., /mnt/c/...

Two gaps stack: the default hints regex has no branches for the Windows-only forms, and resolve_path_for_opening treats POSIX-rooted text as a relative path and joins it onto the OSC 7 cwd.

What this PR does

Detection (rio-backend/src/config/hints.rs): DEFAULT_URL_REGEX is now cfg-split. The Windows const keeps the shared alternations and adds three Windows-only branches — drive-letter paths (both separators), backslash UNC, and the MSYS forward-slash UNC spelling. All three require a dotted filename segment (mirroring the conservative bare-relative branch) and allow spaces and parentheses inside segments (Program Files (x86)). The non-Windows const is byte-for-byte unchanged.

Resolution (frontends/rioterm/src/hints.rs): resolve_path_for_opening gains a cfg(windows) step that converts POSIX-rooted text (/x/..., /cygdrive/x/..., /mnt/x/..., //host/share/...) to a Windows path and accepts it only if it exists, before the existing cwd-join fallback. OSC 8 file:// hyperlink clicks route through the same resolver, so MSYS-style OSC 8 links benefit too.

Deliberately not converted: multi-letter roots such as /tmp (the terminal cannot know which POSIX root they map to) and a bare single-letter /x (cd /d flag is ambiguous prose; ls /d/ with a trailing slash still converts).

Behavior matrix

Path form	Needs shell cwd?	Works in
C:\x\f.txt, C:/x/f.txt, \\srv\sh\d.pdf, //srv/sh/d.pdf	no	any shell
/c/..., /cygdrive/c/..., /mnt/c/...	no	any shell echoing them
relative (README.md, ./x)	OSC 7 (unchanged)	shells with shell integration

Testing

• unit tests for the posix_to_windows decision table (single/multi-letter roots, explicit mount syntax, UNC, double separators, bare-flag rejection)
• unit tests pinning the Windows regex branches (forms above, Program Files (x86), plus prose rejections: option C: drive, versions/timestamps)
• non-Windows DEFAULT_URL_REGEX unchanged; existing suites pass (rio-backend 25/25 hints, rioterm 18/18 hints on Windows)