Add the rust_sliding_sync option and a Rust SlidingSyncHandler

Phase 0 of the sliding sync port (renders/sliding-sync-rust-plan.md). No
behaviour change: nothing calls the Rust handler yet.

- `experimental_features.rust_sliding_sync: off | on`. There is no shadow
  mode. Booleans are accepted, as for `rust_sync`.
- `synapse_rust.sliding_sync.SlidingSyncHandler`, built by `RustHandlers`
  on the native driver only. It shares the v2 sync handler's `SyncStore`
  and carries the pool, deferred and blocking helpers the later phases'
  entry points use.
- `SlidingSyncStats`, observed per request into the
  `synapse_sliding_sync_rust_*` histograms. `SyncDbStats` now counts the
  transactions a reader begins.
- The `synapse_sliding_sync_rust_rooms_total` and
  `synapse_sliding_sync_rust_requests_total` counters.
- `SYNAPSE_TEST_RUST_SLIDING_SYNC` and `_STRICT` for trial, with the handler
  wrapped in `_BlockingRustCalls`, and `RUST_SLIDING_SYNC` for Complement.

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

scripts-dev/complement.sh

@@ -376,6 +376,12 @@ main() {
     export PASS_SYNAPSE_RUST_SYNC="$RUST_SYNC"
   fi
 
+  if [[ -n "$RUST_SLIDING_SYNC" ]]; then
+    # Set `experimental_features.rust_sliding_sync` (off or on). The Rust
+    # sliding sync handler needs RUST_DRIVER=1.
+    export PASS_SYNAPSE_RUST_SLIDING_SYNC="$RUST_SLIDING_SYNC"
+  fi
+
   if [[ -n "$UNIX_SOCKETS" ]]; then
     # Enable full on Unix socket mode for Synapse, Redis and Postgresql
     export PASS_SYNAPSE_USE_UNIX_SOCKET=1