Slice: Export ReadingEnvelope identity for app consumers

[flyingrobots]

Slice

Expose the minimal Echo reading identity jedit needs through the app-facing boundary.

Done when:

• jedit can consume reading id, query/operation identity, basis digest, aperture digest, observer plan identity, retained evidence posture, witness refs, budget posture, rights/posture where available, and residual posture;
• the exported shape remains generic and contains no Jim nouns.

Goalpost

Part of Goalpost: Jim Evidence Gate for Echo v0.1.0 under milestone v0.1.0 - Jim Evidence Gate.