C-D21-NARROWED

Signature-narrowed oracle: fixing a target signature pins the squared-speed multiset up to the quadratic-residue scale action, so all matching covers are found by scanning the scale half-group in cost linear in p. Measured to terminate under 0.35s on D20-hard primes up to 2267 where full enumeration was blocked; 3-valued with replayable certificates, UNSAT-without-proof rejected, timeout OPEN. Removes the D20 per-query wall

In plain language

Instead of listing every cover then filtering, D21 uses the target fingerprint to look only where a match could be, fast even at the hard primes.

Exact statement

Signature-narrowed oracle: fixing a target signature pins the squared-speed multiset up to the quadratic-residue scale action, so all matching covers are found by scanning the scale half-group in cost linear in p. Measured to terminate under 0.35s on D20-hard primes up to 2267 where full enumeration was blocked; 3-valued with replayable certificates, UNSAT-without-proof rejected, timeout OPEN. Removes the D20 per-query wall