test(js): cover sse root dispatch

Cover data-slhx-sse EventSource setup, slhx/message event handling, root-scoped EffectBatch application, and SSE error emission.

req: push/001

req: push/003

req: runtime/001

slhx-js/tests/runtime.rs

@@ -128,6 +128,20 @@ fn runtime_refuses_partial_updates_on_fingerprint_mismatch() {
     assert!(source.contains("if (expected && String(batch.fingerprint) !== expected)"));
 }
 
+#[test]
+fn runtime_applies_sse_effect_batches_inside_roots() {
+    // req: push/001 req: push/003 req: runtime/001
+    let source = slhx_js::RUNTIME_JS;
+
+    assert!(source.contains("const sseSources = new WeakMap()"));
+    assert!(source.contains("const url = root.getAttribute(\"data-slhx-sse\")"));
+    assert!(source.contains("new EventSource(new URL(url, location.href).href)"));
+    assert!(source.contains("source.addEventListener(\"slhx\", (event) => applySseMessage(root, event))"));
+    assert!(source.contains("source.addEventListener(\"message\", (event) => applySseMessage(root, event))"));
+    assert!(source.contains("applyBatch(bytes.buffer.slice(bytes.byteOffset, bytes.byteOffset + bytes.byteLength), root)"));
+    assert!(source.contains("emit(root, \"slhx:sse-error\", url)"));
+}
+
 #[test]
 fn runtime_page_swaps_lowered_slot_ids() {
     // req: wire/001