self-audit: scope the shared-loopback row check to the zero-credit regression signature

In no-secret mode the resolved IP is the shared loopback, so a concurrent
keyless snip's legitimate billable row could fail the zero-credit assertion
falsely. The fallback now counts only credits_used=0 rows (the log-only
contract's actual regression signature); the isolated-IP harness keeps the
strict any-row assertion.