XY-832: [ELF memory P1] Add scoped core memory blocks with archival separation

[yvette-carlisle]

Summary

• Add Postgres-backed core memory blocks, explicit attachments, and append-only audit events.
• Add HTTP readback/admin attachment endpoints and an MCP read-only attached-block tool.
• Update the v2 memory service spec and coverage for scope isolation and archival separation.

Verification

• cargo make fmt
• cargo make lint-fix
• cargo make checks
• semantic drift audit reviewed against implementation anchors