fix: align checkpoint contract to python parity

[rlippmann]

Summary

• Align checkpoint wire shape to Python/spec (checkpoint_version, authoritative_state, pending).
• Serialize pending continuation as replacement payload with prompt_to_user.
• Enforce use_only replacement contract with old_item: null.
• Add focused checkpoint continuation parity coverage, including invalid import atomicity.

Why

Checkpoint continuation parity required exact Python/spec contract matching so restored confirmation flows are deterministic and resumable across host boundaries.

Scope checklist

• Diff is minimal and scoped to the requested behavior
• Python/spec parity is preserved (or mismatch is explicitly documented)
• Fixture semantics are respected (exact string vs non-empty null rule)
• No unrelated refactors
• Relevant tests were run

Notes

• Validation run: npm test (all tests passing).
• No fixture files were modified.