`FederationSender` makes a `RustPerDestinationQueue` for each
destination at the new `queues` level. It forwards the queue methods to
Rust and provides the hooks: catch-up, the sticky backlog, the device
EDUs, and recording a sent transaction (stream positions, the module
callback, the metrics).
The catch-up loop moves into `_CatchUpLoop`, which both queues share.
`StickyEventBacklogTracker.load_backlog` splits the database part out of
`prepare_transaction`.
The sender, catch-up and sticky event tests also run at this level, with
every reactor advance waiting for the Rust loops to settle.
`test_send_receipts_thread` is skipped there: it depends on coroutine
order, which the Rust loop changes.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01R5b7Ruy4cgt27Mjq6Ptxbb
