Implement the Echo adapter and shared conformance suite

[flyingrobots]

Outcome

Integrate Keep beneath echo-cas through the accepted transaction boundary and prove both systems agree on exact logical bytes and recovery outcomes.

Dependencies

Blocked by #17, #19, #20, and #22.

Acceptance criteria

• Echo depends on a released or exact pinned Keep revision through a narrow adapter.
• Keep's public core remains free of Echo types and semantics.
• Echo and Keep agree on exact content identities through shared golden vectors.
• Stage, verify, retain, and Echo admission follow the accepted order.
• Kill/restart tests cover every cross-system crash boundary.
• Orphaned physical material is detected and reconciled without inventing Echo history.
• Missing, corrupt, stale-generation, and ambiguous Keep outcomes remain distinguishable through Echo.
• Existing Echo in-memory/reference tests remain available and deterministic.
• The adapter introduces no per-chunk process spawning, Node sidecar, or hidden whole-blob allocation.
• Benchmark evidence records cross-layer latency, sync count, bytes written, and recovery cost.

Non-goals

• Graft workspace integration.
• Automatic migration from git-cas.
• Cross-machine replication.