D103 — Clearance continuation ramp with warm starts

Decision

Clearance continuation: --ramp-from C0 --ramp-step S --ramp-rounds N [--ramp-cycle K --ramp-stall M] runs the portfolio rounds under clearances lowered by (board - c) for c = C0, C0+S, … below the board's, each stage warm-started from the previous routes that are legal under its rules (--state-recheck: legal against fixed copper, then greedy vertex cover of the conflict graph, then the strict exact check); the last stage and the returned result are always under the board's rules, and with --ramp-cycle the last stage can go back K stages when it stalls ("breathing")

Alternatives

Route straight at the board's rules (plateau at 1–6 open on six srj20 boards after hours); tighten the rules in the router itself without staged warm starts

Reason

At the board's rules the search from scratch stalls although solutions exist (sample048 completes at 0.085 mm when warm-started from the 0.06/0.07 solutions, 30 min from scratch at 0.08 leave one open); the solution at 0.085 uses only 4 double-used via gaps and 43 % of the top pad gaps, so the plateau is search, not capacity. Test: clearance ramp (two cases) in tests/test_txn.cpp