Slice: Define observer coordinate B/A/P protocol shape #60

Slice

Turn the observer-coordinate design into Continuum-owned protocol vocabulary.

Source detail:

• C = (B, A, P)
• B: basis/frontier causal coordinates
• A: aperture/focus window over slots, variables, or event indices
• P: revelation posture such as opaque, redacted, decrypted, or ZK-certified

Done when:

• Continuum names an ObserverCoordinate or equivalent profile shape;
• the shape composes with ObserverPlan, ObservationRequest, and ReadingEnvelope;
• basis/frontier, aperture, and posture are separately digestible;
• participants can reference the coordinate without inventing repo-local names.

Parent: #58.