S2-01 — Semantic checkpoints

Status: Ready Goal: Persist workflow checkpoints only at the semantic boundaries named in ADR-0003.

Acceptance criteria

• Checkpoints append at plan, patch, gate, interrupt, and terminal boundaries.
• Golden ordering test is deterministic.
• Payload size remains bounded.

TDD plan

Red: golden checkpoint-order test. Green: append logic. Refactor: share canonical serialization helpers.