fix(composition): reserve the data-hypit-* attribute namespace

packages/composition/src/track.ts

@@ -580,6 +580,12 @@ function assertAttributes(attributes: readonly VisualAttribute[] | undefined, la
     if (!/^(?:data-[a-z0-9-]+|aria-[a-z0-9-]+|role|title|lang|dir)$/u.test(attribute.name)) {
       throw new Error(`${label} contains unsafe attribute ${attribute.name}.`);
     }
+    // `data-hypit-*` attributes carry compiler-owned semantics (deferred resources, sampling
+    // state, element identity). Authored attributes serialize first and duplicate names keep
+    // the authored value, so an author could otherwise choose what the runtime promotes.
+    if (attribute.name.startsWith("data-hypit-")) {
+      throw new Error(`${label} contains reserved attribute ${attribute.name}.`);
+    }
     if (attribute.name === "lang" && !/^[A-Za-z0-9]+(?:-[A-Za-z0-9]+)*$/u.test(attribute.value)) {
       throw new Error(`${label} contains invalid lang ${attribute.value}.`);
     }

packages/composition/test/track.test.ts

@@ -139,6 +139,21 @@ test("VisualTrack style values cannot smuggle a second declaration", () => {
   );
 });
 
+test("VisualTrack elements cannot claim the reserved data-hypit attribute namespace", () => {
+  const { timeline, visual } = fixture();
+  const first = visual.presents[0]!.elements[0]!;
+  const hostile = sealVisualTrack({...visual,
+    presents: [{
+      ...visual.presents[0]!,
+      elements: [{ ...first, attributes: [{ name: "data-hypit-resource-src", value: "https://invalid.example/x" }] }],
+    }],
+  });
+  assert.throws(
+    () => assertVisualTrackIdentity(hostile, timeline),
+    /reserved attribute data-hypit-resource-src/,
+  );
+});
+
 test("VisualTrack cannot silently extend the versioned public Visual IR", () => {
   const { timeline, visual } = fixture();
   const first = visual.presents[0]!.elements[0]!;