Round 13 follow-up: on Windows Path('/etc/passwd').is_absolute()
returns False because POSIX absolute paths read as drive-relative,
which let the traversal check fall through to resolve(strict=True)
and crash with a raw FileNotFoundError instead of the friendlier
RuntimeError. Add a PurePosixPath check + explicit leading-separator
guard and wrap the resolve() in try/except so a missing path inside
the chosen repo is reported as 'Local repo path does not contain ...'
on every OS.
Pre-existing 59 diffusion backend + route tests still pass; staging
Windows Diffusion CI was failing on this exact case.
54adfdff53