fix(worldmodel): compile on non-unix targets (Windows workspace build)

bridge.rs imported tokio::net::UnixStream unconditionally, so the whole
workspace failed to build on Windows (E0432) — blocking cargo test
--workspace and the pre-merge gate there. The OccWorld Unix-socket bridge
is a Linux-appliance feature (Python inference server on the GPU host), so
gate it #[cfg(unix)] and add a #[cfg(not(unix))] send_recv that fails fast
with a clear 'unsupported on this target' Protocol error. Workspace now
builds on Windows; worldmodel 12 tests pass.

Co-Authored-By: claude-flow <ruv@ruv.net>