refactor(canvas): review fixes for the busy flag, the comparison note and the bundle size check

- busyReason had no reader: the chat engine sets isBusy through the store's
  generic set, like every other flag.
- The isSameDocument comment says the two "same document" rules disagree
  in both directions, so neither is a subset of the other.
- The import boundary test bundles minified with one pako (the MCP core
  resolves its own copy) and caps the canvas core at 160 KB (134 KB now).

components/chat/chat-engine.tsx

@@ -729,7 +729,7 @@ export function ChatEngineProvider({
     // Canvas components (version cards, the compare dialog, "ask AI") read
     // the busy flag from the canvas store
     useEffect(() => {
-        useCanvasStore.getState().setBusy(isBusy ? "chat" : null)
+        useCanvasStore.getState().set({ isBusy })
     }, [isBusy])
     // The page goes (another language mounts a new one): the answer stops,
     // so it cannot reach the next page's canvas