Slice: Specify ZK capability presentation posture

[flyingrobots]

Slice

Define how a ZK capability presentation is represented at the Continuum protocol boundary.

Source detail:

• actor proves knowledge of a valid delegation chain for a target ability/resource;
• intermediate delegation keys and pairwise identifiers may remain hidden;
• public claim should identify target resource, target ability, and proof reference/posture.

Done when:

• public claim fields and hidden witness assumptions are named;
• verified, unverified, descriptor-only, redacted, and credential-required states are representable;
• presentation posture composes with evidence posture;
• tools can explain redacted/private/shared claims without exposing private authority chains.

Parent: #58.