Merge erikj/rust-sss-queries into the Rust event persistence branch

Brings Rust sliding sync (erikj/rust-sss-queries) together with Rust
event persistence (erikj/rust-persist-validate). Both branch from
a78b02b55c.

The conflicts were both sides adding parallel entries:
- the Complement and test environment switches
- the `RustHandlers` fields and getters
- the `_BlockingRustCalls` types

These keep both. `RustHandlers` builds the persister and the sliding sync
handler in one pass over the native pool. `SyncReader::begin` keeps the
persist side's flag-before-BEGIN and the sliding sync side's transaction
count and `begin_write`. Both sides had added the `invite_room_state`
and `knock_room_state` accessors to `Unsigned`, so one copy is kept.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PVLsNh6akt4ifEizY4D1ta

scripts-dev/complement.sh

@@ -386,6 +386,11 @@ main() {
     fi
     export PASS_SYNAPSE_RUST_PERSIST="$RUST_PERSIST"
   fi
+  if [[ -n "$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