backend: prepare durable conversation subscriptions

[sandersonstabo]

Dependency

Directly depends on #325 (Post-write Server Dispatch Receipt).

Scope

• Prepare fresh subscriptions from one durable bounded snapshot read.
• Validate resume cursors through durable bounded patch replay.
• Register only Pending state at the exact response-declared cursor.
• Return lossless resnapshot-required and typed repository/registration failures without partial mutation.
• Make unsubscribe acknowledgement idempotent while retaining the exact removed/absent registry outcome.

Exclusions

No request-handler wiring, post-response activation, notification loop, delivery task/queue, QUIC stream ownership, startup/custody work, or frontend projection.

Verification

• //tests/backend:conversation_subscription_preparation_test (9 Rust cases)
• fresh production and test Clippy aspect markers
• root format test
• locked/offline Cargo metadata
• exact seven-path hash/status guards and shutdown 0

Gate receipt: D:/artisan-editor-gate-logs/backend-conversation-subscription-preparation-20260829-gate2/receipt.json.

---

^{Need help on this PR? Tag @codesmith-bot with what you need. Autofix is disabled.}

---

Summary by cubic

Adds the durable preparation seam for conversation subscriptions. Fresh and resume subscribes now register Pending at exactly the response-declared cursor, and stop acknowledgements are idempotent.

• Durable reads always precede registry mutation, so repository or register failures leave existing state unchanged.
• Fresh subscribes read one bounded snapshot; resume subscribes validate the cursor through one bounded patch replay.
• Resume cursors beyond the durable tail return ResnapshotRequired with both requested and current cursors and make no registry change.
• Prepared entries are only ever Pending; activation is the caller’s responsibility after the Subscribe response write is proven.
• Stop returns the exact Removed or Absent registry outcome and never fabricates a removal.

^{Written for commit ab5109a. Summary will update on new commits.}