[codex] Clarify napkin math plan reset

[neoneye]

Summary

• Reframes the napkin_math 2026-05-22 plan around the v58 corpus review.
• Documents the observed v49 -> v58 regressions: prior signal loss, threshold coverage collapse, and non-operational prior-ledger wiring.
• Replaces the open-ended next-step list with a finite path focused on regression gates before resuming Phase 5 / 8 / 9 work.

Validation

• Docs-only change; no tests run.