[CA-01-S4] Publish an external causal-anchor admission contract for Jim

[flyingrobots]

Goal

Expose enough canonical evidence for Jim to consume real Echo anchor admission without reimplementing Echo identity.

Acceptance criteria

• Public Echo types expose the request, canonical anchor, receipt, and recovered admission.
• A deterministic external-consumer fixture proves subject, basis, roots, purpose, anchor id, and receipt parity.
• Golden or codec evidence prevents Jim from inventing a second anchor digest/domain.
• Documentation identifies the trusted API and explicitly quarantines value-only constructors.
• CHANGELOG records the new externally meaningful authority boundary.

[coderabbitai]

🔗 Related PRs

#323 - Witnessed suffix admission evaluator [merged]
#344 - feat(wesley): consume canonical requirements artifacts [merged]
#358 - feat(core): add admission evidence boundaries [merged]
#529 - Add Hello Echo demo and docs lifecycle policy [merged]
#582 - docs: define the Echo 1.0 release contract and move live planning to GitHub [merged]

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[flyingrobots]

Implemented in 60c5bce and hardened in 75ac3b2. Public opaque request/fact/receipt/recovery types, a Jim-shaped external-consumer witness, and a golden vector expose Echo evidence without exporting identity constructors or inviting a second digest domain.