Send the sticky event backlog in Rust on the native driver

`NativeHooks::sticky_backlog` loads a backlogged batch, cleaning up rooms
whose sticky events expired, reads the events through the shared event
cache and filters them for the destination. Its completion records the
batch natively. Where the filter would wait for full state, the batch is
left to Python, as before.

`FederationStore` also gains `get_catch_up_outstanding_destinations` for
phase 6. Each query has a differential test against Python's.

The native queue tests now run with MSC4354 on, so the loops look at the
sticky event backlog, and pin that it takes no GIL.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01R5b7Ruy4cgt27Mjq6Ptxbb

rust/src/federation/queue/python.rs

@@ -550,7 +550,11 @@ impl QueueHooks for PyHooks {
         .await
     }
 
-    async fn sticky_backlog(&self, observer: &Observer) -> Result<Option<StickyBatch>, LoopError> {
+    async fn sticky_backlog(
+        &self,
+        _queue: &DestinationQueue,
+        observer: &Observer,
+    ) -> Result<Option<StickyBatch>, LoopError> {
         self.call(
             observer,
             "queue_sticky_backlog",