#28063 fixed the macOS `/tmp`→`/private/tmp` symlink issue by checking
the RAW path (pre-resolve) against startswith('/tmp/'). That works on
Linux + macOS but not on Windows — Path('/tmp/foo').resolve() returns
C:\\tmp\\foo and isn't the real Windows temp anyway.
Replace the hardcoded '/tmp/' prefix with Path(tempfile.gettempdir()).
resolve() + Path.relative_to() — same idiom as the cwd branch just
below. Works correctly on Linux (/tmp), macOS (/private/var/folders/...),
and Windows (%LOCALAPPDATA%\\Temp).
Test rewritten to use tempfile.gettempdir() so the assertion exercises
the same code path on every platform.
Conflict against the just-merged #28063 (raw_path approach) resolved
by replacing the whole raw_path block — tempfile.gettempdir() is
strictly better than that intermediate fix.
Salvage of #28262 by @Zyrixtrex.
