Backend Conversation Subscription Registrar

[sandersonstabo]

Depends on #329.

Scope

• Extracts the request handler subscription table behind a narrow cloneable ConversationSubscriptionRegistrar.
• Allows the future connection-local delivery owner to publish post-send batches through the same registry.
• Preserves handler-private receipt identity, lease fencing, stale-lease behavior, and current request behavior.

Exclusions

• No writer queue, notifier loop, listener/startup composition, frontend routing, or packaging changes.
• No cross-process data-directory custody decision.

Verification

• //tests/backend:request_handler_test (46 Rust cases)
• fresh production and integration-test Clippy markers
• root //:format_test
• locked/offline Cargo metadata
• exact seven-path hash/status/diff guards
• Bazel shutdown exit 0

Authoritative receipt: D:/artisan-editor-gate-logs/backend-conversation-subscription-registrar-20260829-gate4/receipt.json.

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Summary by cubic

Extracts the request handler's subscription table into a cloneable ConversationSubscriptionRegistrar so a future connection-local delivery owner can record published batches through the same registry. Request behavior, receipt identity, and lease fencing are unchanged.

Refactors

• RequestHandler::with_subscriptions now creates a fresh registrar; new with_subscription_registrar accepts a shared one.
• record_published_batch applies a batch only after wire publication and enforces the same lease, thread, state, and cursor fences.
• Receipt identity stays outside the registrar, so a clone cannot activate another handler's receipt.

^{Written for commit 6fd9d33. Summary will update on new commits.}