refactor(observation): isolate document processing from capture orchestration

apps/extension/src/tools/vom/capture-abort.ts

@@ -0,0 +1,32 @@
+function captureAbortError(): Error {
+  const error = new Error("observation aborted");
+  error.name = "AbortError";
+  return error;
+}
+
+export function isAbortError(error: unknown): boolean {
+  return (
+    typeof error === "object" &&
+    error !== null &&
+    (error as { name?: string }).name === "AbortError"
+  );
+}
+
+export function throwIfAborted(signal: AbortSignal | undefined): void {
+  if (signal?.aborted) throw captureAbortError();
+}
+
+/** Called only at bounded block boundaries, not once per node. */
+export async function captureCheckpoint(signal?: AbortSignal): Promise<void> {
+  throwIfAborted(signal);
+  await new Promise<void>((resolve) => {
+    const channel = new MessageChannel();
+    channel.port1.onmessage = () => {
+      channel.port1.close();
+      channel.port2.close();
+      resolve();
+    };
+    channel.port2.postMessage(null);
+  });
+  throwIfAborted(signal);
+}