- Split DSH_COVERAGE_MAX_WORKERS between the two parallel gates
(instrumented gets ~2/3, exempt gets ~1/3, both at least 1) so the
lane never exceeds the budget the failover pool's 8 x 6-instance
bound assumes; a budget of 1 pairs with DSH_GATE_CONCURRENCY=1 on
the serial reference lanes, which already prevents gate overlap.
- Fail loud on a set-but-not-'1' DSH_COVERAGE_EXEMPT_HEAVY value in
vitest.config.ts instead of silently ignoring it.
- Add coverage-exempt.spec.ts: each roster entry's filter and exclude
must select the same non-empty spec set and entries must not
overlap, so a renamed suite breaks the gate instead of silently
returning to the instrumented run with a stale roster.
