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.
Problem: a TU skipped because a prior run quarantined it was not analysed,
but coverage did not say so. The serial path dropped it from the attempted
set, so a scan whose only TU was quarantined was complete and exited 0; the
fan-out path saw no file_finished record and called it frontend-failed.
Fix: the shard runner counts every requested TU and records skipped ones
with gap reason `quarantined`; a worker emits a file_finished record with
that reason so the parent classifies it the same way. USAGE lists the reason.
Test: test_subprocess_fanout "[quarantine]" covers serial (was 2 attempted,
no gap) and fan-out (was frontend-failed). CLI probe: serial and fan-out
both exit 2 with a `quarantined` gap.
