Migrate durable rewind checkpoints to Runtime HBP

[wesleysimplicio]

Summary

• persist durable RewindCheckpoint blobs as one Runtime-compatible HBP v1 record
• migrate legacy checkpoint-.json only during bounded startup rehydrate, preserving a .legacy.bak and failing closed on corrupt or mismatched input
• add focused migration, tamper-shape, cap, truncate, and rehydrate tests; classify the exact JSON boundary as migrated
• repair the existing test SessionContext factory for the required directory field

Evidence

• Runtime MCP: cargo check -p xai-grok-workspace: PASS
• Runtime MCP: cargo fmt --all -- --check: PASS
• Runtime MCP: cargo test -p xai-grok-workspace checkpoint_store: 13 passed, 0 failed
• Runtime MCP: cargo test -p simplicio-code-formats --lib: 8 passed, 0 failed
• Runtime MCP: strict binary boundary scan: findings=0, pending=0, expired=0
• git diff --check: PASS

Closes #99