feat(sync): add typed broadcast primitive

req: sync/004

examples/kanban/tests/browser_e2e.rs

@@ -2200,6 +2200,54 @@ async fn adversarial_wire_inputs_are_rejected_before_partial_application() -> We
     result.and(quit)
 }
 
+#[tokio::test]
+async fn typed_broadcast_applies_generated_batch_over_sse() -> WebDriverResult<()> {
+    // test req: sync/004
+    let app_port = available_port();
+    let app_addr = format!("127.0.0.1:{app_port}");
+    let mut app_command = Command::new(env!("CARGO_BIN_EXE_hemx-kanban-example"));
+    app_command.env("HEMX_KANBAN_ADDR", &app_addr);
+    let _app = TestProcess::start(app_command, "hemx-kanban", &app_addr, STARTUP_TIMEOUT)
+        .expect("start hemx-kanban");
+
+    let webdriver_port = available_port();
+    let webdriver_addr = format!("127.0.0.1:{webdriver_port}");
+    let mut webdriver = Command::new("geckodriver");
+    webdriver.arg("--port").arg(webdriver_port.to_string());
+    let _webdriver = TestProcess::start(webdriver, "geckodriver", &webdriver_addr, STARTUP_TIMEOUT)
+        .expect("start ready geckodriver");
+    let mut caps = DesiredCapabilities::firefox();
+    caps.set_headless()?;
+    let driver = WebDriver::new(&format!("http://{webdriver_addr}"), caps).await?;
+
+    let result = async {
+        driver.goto(&format!("http://{app_addr}/")).await?;
+        wait_until(
+            &driver,
+            "return document.body.textContent.includes('tick #7')",
+        )
+        .await?;
+        let resources = driver
+            .execute(
+                "return performance.getEntriesByType('resource').map(entry => entry.name).filter(name => name.includes('/sync/broadcast'))",
+                Vec::new(),
+            )
+            .await?
+            .json()
+            .clone();
+        assert!(
+            resources
+                .as_array()
+                .is_some_and(|resources| !resources.is_empty()),
+            "typed broadcast SSE request was not observed: {resources}"
+        );
+        Ok(())
+    }
+    .await;
+    let quit = driver.quit().await;
+    result.and(quit)
+}
+
 #[tokio::test]
 async fn ordinary_browser_request_exposes_deadline_and_cancels_on_pagehide() -> WebDriverResult<()>
 {