D44 — Flood-fill reachability pre-check before strict searches

Decision

A flood-fill reachability pre-check runs before strict searches that are likely to fail (a retry with a larger window, or a connection that failed before); "no path" skips the A* and its cost-to-go field (--reach-check 1, default; 0 off, 2 every strict search; --reach-verify re-checks with the full A*)

Alternatives

Reusing the failed search's explored region on window growth; a coarse-grid check; checking every strict search

Reason

The flood uses the same legality tests with no bend states or costs, so it admits every A* path and its verdict is exact (0 mismatches under --reach-verify). 10 hard boards, one variant, 100 M: 8,285 → 8,332 routed, no board worse; every-search mode 8,336 but swings ±45 per board. Tiers A–D neutral (2,637 / 5,165 / 9,523 / 16,628 routed, clean pass unchanged). Region reuse would not keep the A* optimal in the larger window; a coarse grid would not be exact