Slice: Return retained receipt and reading evidence refs

[flyingrobots]

Slice

Lift retained evidence references into the app-facing witness path for receipts and readings.

Done when:

• jedit can inspect retained receipt and reading-envelope refs;
• missing retained material is explicit posture;
• semantic refs are not replaced by raw content-hash guesswork.

Goalpost

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

[coderabbitai]

🔗 Related PRs

#371 - feat(core): advance v0.1 contract evidence and retention [merged]

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