`rust_sync: on` was refused unless `rust_sync_allow_unshared_event_cache`
was set, because the Rust sync handler held a second copy of every event
it served. Python and Rust now share one event cache, so the gate and its
option go.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VpBQxvqZTUvHCMm3u6CR7J