fix(preview): hung-frame watchdog + require approval for guessed action calls

A runaway preview script (sync infinite loop, or a re-render loop
outpacing the bootstrap's own coalescing) had nothing detecting it -
the frame just spun. The bootstrap now heartbeats every second, and the
parent tears the iframe down if it goes _WATCHDOG_MS silent, whatever
the cause.

_coerce_tool_calls recovers a tool call guessed from `content` for
models with no native tool_calls field. That guess is weaker evidence
than the API's own structured field - a model can land on JSON shaped
like a call while only meaning to describe one - so an action tool
recovered this way now always requires approval, even under the
"allow" policy that lets a native tool_calls field run unattended.

interface/web/src/Markdown.jsx

@@ -243,6 +243,9 @@ const _PREVIEW_BOOTSTRAP = `<script>
       observers[observers.length - 1].observe(document.body);
     }
     setTimeout(post, 300); // late paints: fonts, async draws, first rAF frame
+    // Heartbeat: the parent's watchdog needs a message even when nothing is
+    // changing, or an idle-but-alive frame reads the same as a hung one.
+    setInterval(post, 1000);
   });
 })();
 </script>`;
@@ -304,6 +307,14 @@ const _MIN_PREVIEW_H = 160;
 const _MAX_PREVIEW_H = 720;
 const _MAX_H_STEPS = 60;
 
+// A frame that never posts again — a synchronous `while(true)` in the user's
+// own script, or a runaway re-render loop the bootstrap's own coalescing
+// can't outpace — has nothing else to signal it. Silence past this long since
+// mount (or since the last message) is treated as hung and the frame is torn
+// down; the bootstrap's 1s heartbeat means a merely-idle-but-alive frame never
+// gets close to this.
+const _WATCHDOG_MS = 6000;
+
 // Live preview for a renderable fenced block: a Preview/Code toggle rendered
 // via a sandboxed iframe whose document is an encoded data: URL.
 //
@@ -407,9 +418,11 @@ function PreviewFrame({ lang, value, expanded }) {
   const [doc, setDoc] = useState("");
   const [buildError, setBuildError] = useState("");
   const [height, setHeight] = useState(240);
+  const [hung, setHung] = useState(false);
   const frameRef = useRef(null);
   const heightRef = useRef(240);  // mirrors `height` so the listener needn't re-subscribe
   const stepsRef = useRef(0);
+  const lastMsgRef = useRef(0);  // set for real by the watchdog effect below
 
   // Receive the bootstrap's reports. The frame is on an opaque origin, so
   // e.origin is the string "null" and proves nothing - identify the sender by
@@ -419,6 +432,7 @@ function PreviewFrame({ lang, value, expanded }) {
       if (!frameRef.current || e.source !== frameRef.current.contentWindow) return;
       const data = e.data;
       if (!data || data.__nexusPreview !== 1) return;
+      lastMsgRef.current = Date.now();
 
       if (typeof data.err === "string" && data.err) setError(data.err);
 
@@ -435,6 +449,23 @@ function PreviewFrame({ lang, value, expanded }) {
     return () => window.removeEventListener("message", onMessage);
   }, []);
 
+  // Watchdog: a frame that goes silent past _WATCHDOG_MS — most likely a
+  // synchronous infinite loop in the model's own script, which blocks even
+  // the bootstrap's heartbeat from ever running — gets torn down rather than
+  // left spinning. Checked on an interval rather than a single timeout so a
+  // message arriving late (slow compile, heavy first paint) keeps resetting
+  // the clock instead of tripping early.
+  useEffect(() => {
+    lastMsgRef.current = Date.now();
+    const id = setInterval(() => {
+      if (Date.now() - lastMsgRef.current > _WATCHDOG_MS) {
+        setHung(true);
+        clearInterval(id);
+      }
+    }, 1000);
+    return () => clearInterval(id);
+  }, [lang, value]);
+
   useEffect(() => {
     let current = true;
     setDoc("");
@@ -448,9 +479,13 @@ function PreviewFrame({ lang, value, expanded }) {
   }, [lang, value]);
 
   // A build failure (JSX that doesn't parse) has no document to show at all, so
-  // the message stands in for the frame rather than sitting under it.
-  const frameUrl = doc ? `data:text/html;charset=utf-8,${encodeURIComponent(doc)}` : "";
-  const shown = buildError || error;
+  // the message stands in for the frame rather than sitting under it. A hung
+  // frame tears down the same way: dropping frameUrl unmounts the iframe,
+  // which is what actually stops a runaway script from holding the tab.
+  const frameUrl = doc && !hung ? `data:text/html;charset=utf-8,${encodeURIComponent(doc)}` : "";
+  const shown = hung
+    ? "Preview stopped responding (likely an infinite loop) and was stopped."
+    : buildError || error;
 
   return (
     <>