Slice: Specify Verkle IPA aperture opening proof fixture

[flyingrobots]

Slice

Define the first non-production aperture proof fixture for Verkle/IPA-style openings.

Source detail:

• a history committed under root R;
• aperture A selects non-contiguous points from N history events;
• proof compresses openings for selected points rather than shipping the whole causal bulk.

Done when:

• fixture inputs include basis commitment/root, aperture indices, opened value digests, public reading digest, and proof reference;
• the fixture distinguishes descriptor-only, unverified, and verified proof posture;
• no production security claim is made until the proof system is actually verified;
• the fixture can be attached to observation-profile evidence posture.

Parent: #58.