Slice: Add checkpoint materialization boundary witnesses #221

Goal

Prove that optional checkpoint materializations remain projection artifacts while Echo retains sole ownership of anchor authority and policy.

Acceptance criteria

• Jim validates supported materialization roles and rejects malformed or duplicate role/id pairs before invoking Echo.
• Materialization roots are optional and never become rope authority roots.
• Tests prove invalid requests make zero Echo-capability calls.
• Tests prove the injected capability cannot mutate Jim-owned request or admitted-fact state.
• Retention classes, basis selection, support policy, and anchor identity remain Echo-owned and are not reimplemented in Jim.
• Deterministic admission fakes live only in unmistakably test-only code and never claim authority: "echo".