Design diary (traceability)

This diary records design pressures, reversals, and lessons learned. Update it only when something hurts.

• ANN integration: kept a single reference runner to avoid taxonomy sprawl; randomness surfaces must be explicit and audited.
• Replay semantics: deterministic is replayable; ND is envelope-validated only. Storing replayable was removed to prevent drift.
• Capability honesty: backends must declare supports_ann; dishonesty now fails fast with structured errors.
• Retention limits: ledgers enforce artifact/result caps to avoid slow operational leaks.

When making changes, add a short bullet describing the pressure and the chosen trade-off.