Slice: Define canonical digest domains for observation evidence

[flyingrobots]

Slice

Create stable digest domains for command/event-adjacent observation evidence so later crypto has fixed inputs.

Done when:

• coordinate, reading envelope, aperture, retained evidence, and proof reference digests are domain-separated;
• existing QueryReadingIdentity semantics remain stable;
• Jim can cite these digests without knowing Echo internals.

Goalpost

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