F10 SLICE-2: add renderPlain off-state (#29)

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

src/trackChangesModel.ts

@@ -400,6 +400,16 @@ export function renderAuthorship(
     .join("\n");
 }
 
+/** Off-state body: the current buffer as plain markdown, no annotations (INV-33). */
+export function renderPlain(currentText: string, opts: RenderOptions = {}): string {
+  const render = opts.render ?? defaultRender;
+  try {
+    return render(currentText);
+  } catch (err) {
+    return chip(err instanceof Error ? err.message : String(err));
+  }
+}
+
 /** Pure entry point: annotated HTML body for the preview (INV-22). */
 export function renderTrackChanges(
   baselineText: string,