tz-dev/PMS-SEX: v1.0.1 — Structural Closure Alignment

Aligned chapter closures (17–20) to a uniform praxeological closure structure (structural result, cost distribution, viability corridor, entry guard, marker). Corrected Entry Guard formulations in Chapters 18 and 19 to include explicit semantic meaning, not checkbox-style validation. Reworked Chapter 20 closure to function strictly as a structural alarm boundary, removing residual adapter ambiguity. Clarified reversibility conditions under accumulated costs (Θ), without implying reset or undo semantics. No changes to operator definitions, drift chains, or viability criteria.