verify_plan.py: regen-headroom arithmetic check (armed ≥2× re-gen vs max_model_len pin) — third recurrence of the #505/#601 class

Goal

Add a verify_plan.py WARN-class check: for any plan declaring an ARMED ≥2× cap-hit re-gen trigger alongside a max_model_len pin, assert max_model_len − 2×cap ≥ stated prompt-token bound — WARN when the arithmetic is non-positive or the prompt bound is unstated. Demonstrated miss (third recurrence of the #505/#601 cap-raise-vs-max_model_len class): #2221 plan v9 armed the P6 re-gen at regen_cap = 2×2048 = 4096 against build_vllm_engine's hard max_model_len=4096 pin — zero prompt headroom, so the reused _regen_cell mechanism would skip EVERY row as regen_overlong_skipped, write regen_applied: true with n_regen=0, and silently re-commit the parent's flagged cap-hit deviation while the plan claimed to fix it. verify_plan v9 pass: PASS n_fail=0 n_warn=0 — the class is currently invisible to the mechanical checker; the Methodology critic caught it by reading the engine builder and the regen docstring.

Sketch (from the critic's mechanizable note): detect (a) re-gen arming vocabulary ("re-gen trigger ARMED", "regenerate ... at ≥2× the cap", phase_rollouts_regen/_regen_cell mentions) with a numeric cap, (b) a max_model_len (or VLLM_MAX_MODEL_LEN) numeric pin in the same plan, (c) a stated prompt-token bound (e.g. "≤1,900 prompt tokens"); WARN when 2×cap + prompt_bound > max_model_len or when (a)+(b) present with no stated prompt bound. Standalone N/A escape line for incidental mentions (e.g. N/A — no armed re-gen trigger), per the existing check grammar. Include a pin test reproducing the #2221 v9 shape and a clean case (regen engine at 8192).

Provenance

Surfaced as a workflow-fix prose follow-up by the Methodology critic on #2221 plan v9 (2026-08-13, Phase 2 of the specialized_corpus_remine amendment round); routed by the #2221 orchestrator per .claude/rules/workflow-fix-on-bug.md. Target file: scripts/verify_plan.py. Also worth a one-line extension to the gotchas.md cap-raise rule naming the REGEN leg explicitly. Candidate fingerprint: verify-plan-regen-headroom-arithmetic.