Windows: sessions under vanished directories never match --cwd filter

[Drix10]

What happened?

On Windows, txcript list --cwd . hides sessions whose recorded cwd no longer exists. under_dir canonicalizes both sides and compares with
starts_with, but canonicalize prepends \\?\ only when the path still exists: a live dir becomes \\?\C:\... while a deleted session_cwd stays
C:\..., so the prefix check always fails. Forward-slash spellings (C:/...) and case differences (c:\users vs C:\Users) fail the same way on a
case-insensitive filesystem.

Expected: under_dir strips the verbatim-device prefix and compares case-insensitively on Windows, so vanished directories compare as plain components.

How to reproduce

On Windows, from a repo checkout with a session recorded under it, delete (or rename) the session's directory, then run txcript list --cwd . — the
session is missing.

Verified with scratch tests on unpatched main (both fail): a live tempdir paired with a nonexistent child path, and a mixed-case drive-letter path
(c:\Some\Repo\... vs C:\some\repo). Both pass with the fix; the PR adds them as permanent tests.

Versions

• txcript: 0.14.4
• Agents and their versions: n/a (CLI filtering)
• Operating system: Windows