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/backend.rs

@@ -0,0 +1,39 @@
+use std::sync::Arc;
+
+use serde_json::Value;
+
+use crate::pipe::{Action, CommandOutput, ExecutionSurfaceMetadata, PipeError};
+
+pub trait BrowserBackend: Send + Sync {
+    fn invoke(
+        &self,
+        action: Action,
+        params: Value,
+        expected_domain: &str,
+    ) -> Result<CommandOutput, PipeError>;
+
+    fn surface_metadata(&self) -> ExecutionSurfaceMetadata;
+
+    fn supports_eval(&self) -> bool {
+        true
+    }
+}
+
+impl<T: BrowserBackend + ?Sized> BrowserBackend for Arc<T> {
+    fn invoke(
+        &self,
+        action: Action,
+        params: Value,
+        expected_domain: &str,
+    ) -> Result<CommandOutput, PipeError> {
+        self.as_ref().invoke(action, params, expected_domain)
+    }
+
+    fn surface_metadata(&self) -> ExecutionSurfaceMetadata {
+        self.as_ref().surface_metadata()
+    }
+
+    fn supports_eval(&self) -> bool {
+        self.as_ref().supports_eval()
+    }
+}