fix(runtime): restoreLastOutput falls back on an empty frame, matching Ink (#100)

The frame-restore after an external stdout write (console.log / useStdout().write)
used `lastOutputToRender ?? lastOutput + "\n"`. `??` only falls back for
null/undefined, so an empty-string lastOutputToRender (the initial value, and the
value left by the screen-reader empty-frame path) restored "" — nothing — where Ink
restores lastOutput + "\n". Ink uses `||` (ink.tsx:507) and so does vue's own
mountedClear (render.ts:668); :518 was the lone inconsistent site. Changed `??` to
`||`. Locked by an SR-empty-frame + external-write test that re-emits "\n".

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/runtime/src/render.ts

@@ -515,7 +515,13 @@ export function createApp(root: Component, rootProps?: RootProps | null): TuiApp
       // Clear() resets log-update's cursor state, so replay the latest cursor
       // intent before restoring output after external stdout/stderr writes.
       writer.setCursorPosition(cursorPosition);
-      writer.write(frameState.lastOutputToRender ?? frameState.lastOutput + "\n");
+      // Use `||` (not `??`): an EMPTY lastOutputToRender — its initial value before
+      // the first content commit, the value the narrowing-resize path assigns
+      // (render.ts:1043), and what an empty screen-reader frame leaves — must fall
+      // back to `lastOutput + "\n"`, matching Ink (ink.tsx:507) and vue's own
+      // mountedClear (render.ts:668). `??` only falls back for null/undefined, so an
+      // empty string would pass through and restore nothing after an external write.
+      writer.write(frameState.lastOutputToRender || frameState.lastOutput + "\n");
     }
 
     function writeToStdout(data: string) {