Zarankiewicz LP exceptions: neighbouring table cells give free per-line degree bounds

When a shadow/LP certificate for z(m,n;2,2) <= N leaves exception line patterns at N+1 incidences, three deletions using already-proved cells often do most of the work. Point deletion: d_x >= N+1 - z(m-1,n). Line deletion: |b| >= N+1 - z(m,n-1), which kills every pattern with small lines. Deleting a line together with its s points gives sum of degrees on the line >= N+1 - z(m-s,n-1). For z(17,24) <= 91 this forced degree >= 5, size >= 3, and degree excess >= 1 on every quad and quint and >= 2 on every triple. 2 of 6 exceptions died at once. A third (one quint) died by a tight miss/degree count plus one R-P pair count. Also: a gencert-style loop over totals N+1..N+1+2n spends almost all of its time enumerating empty pattern lists. Break as soon as a total past N+1 has no patterns (2 s instead of 40 min).