docs: record adversarial-review and per-state fixture learnings

[levifig]

Reflect updates from shipping the intent and exploration foundation (#122) and the alpha.10/alpha.11 hotfix cycle (#124), approved during /reflect:

• Extend the Adversarial Review operating principle with PR feat: establish the intent and exploration foundation #122 evidence: a Codex adversarial pass after two independent Claude reviews found a state-integrity defect, a schema constraint gap, and eleven further findings; the pass is now recommended for foundational state-model and persistence changes, not only guidance.
• Add a Test Fixture Hygiene rule: one explicit fixture per supported starting state for migration and classification code; the alpha.10 regression shipped through an accepted transitive-coverage gap.