Slice: Map crypto proof posture into observation profile evidence

[flyingrobots]

Slice

Connect the crypto track back to Continuum protocol surfaces instead of leaving it as separate theory.

Done when:

• continuum.observation.v1 and ReadingEnvelope can carry or reference proof posture for coordinate, aperture, observer law, and capability evidence;
• evidence posture distinguishes native Continuum evidence, translated substrate evidence, fixture evidence, descriptor-only proof, verified proof, and unverified proof;
• downstream apps such as Jim can report proof posture without overclaiming;
• runtimes such as Echo can emit compatible evidence references without importing app product nouns.

Parent: #58.