Problem: normalizeClangTuPath, normalizeQuarantinePath and fan-out's
normalizePath were three copies of the same absolute + weakly_canonical
routine. Coverage, worker records and the quarantine file must agree on a
TU's spelling, and three copies can drift apart.
Fix: move the routine to fold/cli/tu_path.hpp as normalizeTuPath and use it
at every site. No behaviour change.
Test: existing test_scan_coverage (path spellings), test_tu_quarantine and
test_subprocess_fanout, updated to the new name. Full suite: 140/140.