feat: add websocket browser service runtime

Wire the service/browser runtime onto the websocket-driven execution path and add the new browser/service modules needed for the submit flow and runtime integration.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/browser/mod.rs

@@ -0,0 +1,19 @@
+pub mod bridge_backend;
+pub mod bridge_contract;
+pub mod bridge_transport;
+pub mod callback_backend;
+mod backend;
+pub(crate) mod callback_host;
+mod pipe_backend;
+pub mod ws_backend;
+pub mod ws_probe;
+pub mod ws_protocol;
+
+pub use backend::BrowserBackend;
+pub use bridge_backend::BridgeBrowserBackend;
+pub use callback_backend::{
+    BrowserCallbackBackend, BrowserCallbackError, BrowserCallbackHost,
+    BrowserCallbackRequest, BrowserCallbackResponse, BrowserCallbackSuccess,
+};
+pub use pipe_backend::PipeBrowserBackend;
+pub use ws_backend::WsBrowserBackend;