feat(backend): add conversation subscription registry

[sandersonstabo]

Depends on #323.

Adds a deterministic per-connection conversation subscription registry with single-copy authority, pending-to-active activation, stale-lease fencing, exact unsubscribe ownership, monotonic generations, and contiguous cursor enforcement for published batches.

Scope:

• backend registry source and focused integration tests
• backend Cargo/Bazel/export registration
• root backend suite visibility

Excludes request-handler/connection injection, QUIC writer ownership, notifier queues, startup replay, and frontend projection.

Verification:

• //tests/backend:conversation_subscription_registry_test — 17/17
• fresh production and integration-test Clippy markers
• root format check
• locked/offline Cargo metadata
• exact seven-path hash/status guards
• Bazel shutdown exit 0

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Summary by cubic

Adds a synchronous per-connection conversation subscription registry that fences writes with per-thread leases and enforces a pending-to-active lifecycle before batches can advance the cursor.

Details

• Tracks one entry per thread with monotonic generation counting; replacing or removing an entry immediately stales its lease.
• publish_batch accepts only an active lease, exact thread match, and a contiguous from_cursor equal to the current cursor, then advances to to_cursor.
• All failures are typed and return without mutating state.
• Excludes request-handler/connection injection, QUIC writer ownership, notifier queues, startup replay, and frontend projection.
• Registers the new module in Cargo and Bazel and adds a 17-case integration test suite.

^{Written for commit 4dc90d6. Summary will update on new commits.}