#216 Replay: Add checkpoint versioning for schema compatibility

## Problem If code changes between runs, old checkpoints may be incompatible with new code. ## Example ```python # Old code: user = ctx.step("fetch_user", lambda: {"id": 1, "name": "Bob"}) # New code: user = ctx.step("fetch_user", lambda: {"id": 1, "name": "Bob", "email": "bob@x"}) # Checkpoint still returns old format without email! ``` ## Impact - Silent data format bugs during replay - No way to detect checkpoint incompatibility - No migration path for cached results ## Solution 1. Add checkpoint version tracking (hash of expected schema) 2. Version incompatibility detection on replay 3. Optional: Migration hooks for checkpoint schema changes ## Effort: Large