Slice: Specify ZK observer-law proof wrapper

[flyingrobots]

Slice

Specify the ZK wrapper around observer-law execution after the aperture proof fixture exists.

Source detail:

• private witness: aperture contents plus observer-law execution details;
• public claim: output digest Y, basis commitment R, and proof that opened aperture values under R executed law L to produce Y.

Done when:

• public inputs and private witness fields are named;
• observer-law identity is canonical and versioned;
• failure/unsupported/redacted proof posture is representable;
• this remains gated behind stable observer-law semantics.

Parent: #58.