workflow-fix: verify_plan.py p-floor attainability check for empirical-null gates

Overview / Motivation

Auto-filed by the workflow-fix-on-bug protocol from a workflow-fix candidate raised on task #816 (emitting agent: codex-critic, statistics lens, plan v5 round 1).

Goal

Add a p-floor attainability check to scripts/verify_plan.py: FAIL when a plan's registered empirical-null success gate requires any null family to pass p ≤ alpha while that family's p-floor (1/(n_draws+1)) exceeds alpha.

Workflow gap

• Bug observed: #816 plan v5 registered an Exp-5 SUCCESS gate "real |r| > every honest null family's 97.5th-pct at one-sided empirical p ≤ 0.05 surviving BH" while the ladder included cross-trait (n_draws=2, p-floor 1/3) and PCA (n_draws=5, p-floor 1/6) families — the gate is structurally unattainable. verify_plan.py c8 checked criteria PRESENCE and passed the plan; only the Codex statistics critic caught it.
• Why it is a workflow gap: the verifier owns mechanical plan checks; attainability of a registered empirical-p gate is computable (parse family n_draws + registered alpha; compare p_floor to alpha) and recurs in every null-battery / permutation-test plan.
• Confidence (emitter): high (mechanizable: yes per the critic verdict).

Proposed change (candidate diff sketch — refine in planning)

+ def check_empirical_gate_attainability(plan_text):
+     # find registered gates of form "p <= ALPHA" tied to null families with
+     # declared n_draws; p_floor = 1/(n_draws+1); FAIL if p_floor > ALPHA for
+     # any family the gate requires to pass; WARN if p_floor == min attainable

Scope / surfaces

• Primary target: scripts/verify_plan.py
• Grep the workflow surface for sibling checks before editing (grep -rln 'p_floor\|empirical p' scripts/verify_plan.py .claude/) and add a matching test in tests/test_verify_plan.py.

Constraints / invariants

• Workflow-surface only. scripts/workflow_lint.py --check-asks passes; ruff passes; tests added for the new check (positive + N/A escape).
• This is NOT architectural (adds a check; no schema/CLI contract change).

Provenance

• workflow_fix_target: scripts/verify_plan.py
• fingerprint: 76131138e355

Origin (verbatim critic prose): "mechanizable: yes; check every family included in the registered BH/test set, read n_draws, compute p_floor=1/(n_draws+1), and fail if p_floor exceeds the registered alpha while the family is required to pass p<=alpha. This belongs in a recurring workflow-surface verifier for registered empirical-null corrections." (epm:plan-critique-codex v1 lens=statistics, task #816, 2026-07-02)