Retain independent Echo BlobHash conformance evidence #780

1. Background Context

The actual Echo–Keep bridge already executes Keep and validates reconstruction. Old PR #723 also retained independently generated raw Echo hash vectors and a correction to the canonical Keep contract link. Preserve those useful evidence anchors before closing that superseded proposal.

2. Problem Description

Current actual bridge coverage anchors the raw Echo empty vector but recomputes its nonempty Echo expectations through the same implementation. The canonical boundary also links to a nonexistent path at the pinned Keep commit; the invariant README exists at that exact commit.

2b. Proposed Solution

Retain the empty/text/256-byte-ramp raw Echo golden expectations in echo-cas, without copying a Keep algorithm into a mock witness. Correct only the pinned link's path.

2c. Alternatives considered and rejected

Merge #723 wholesale: duplicates the stronger actual Keep bridge and introduces an obsolete same-code witness.
Drop the old proposal entirely: loses useful independent nonempty Echo expectations and a valid link repair.

2d. Acceptance Criteria

• Actual blob_hash matches all three preserved raw vectors.
• The canonical Keep URL names a file present at its unchanged pinned commit.
• Existing Echo/Keep identity and experimental adoption boundaries remain accurate.
• Bind Echo and Keep content identities explicitly #723 closes only after its useful independent evidence is integrated with a traceable replacement.

2e. Test Plan

Golden: three fixed raw BLAKE3 expectations retained from #723 at db3e5df; original provenance reports b3sum 1.8.5, not a new generator run.
Edges: empty bytes, text including newline, every byte value.
Known failure modes: changed hash framing, lost fixture bytes, wrong canonical document path.
Fuzz and stress: none needed; bounded 0/18/256-byte fixtures.

3. Prerequisites

• feat(keep): verify the experimental Echo content identity bridge #768 actual Keep identity bridge is merged, so the older mock witness can retire without losing the real crossing.
  The agreed landing order places this salvage after corrected Continue retained native operation strands across host recovery #728; this is sequencing, not a correctness dependency.

4. Scope

In: independent raw Echo vectors and pinned canonical contract URL.
Out: Keep algorithm reimplementation, storage behavior, hash/durable format changes, backend adoption and old proposal's unsupported broader claims.

5. Why now

Complete the agreed #723 salvage and close the obsolete PR with evidence preserved.

6. Risks

Overstating independent generation or current Keep execution. Label the retained fixture provenance and keep actual bridge validation separate.

7. Definition of Done

A small independently reviewed PR passes guarded CAS tests/Clippy/fmt and hosted checks, lands on main, and is linked from #723 before closure.

8. Stakeholders

Echo CAS and experimental Keep bridge maintainers.

9. Related Issues

PR #723; merged #768 and #770; tracking container #722.