fix(exam): stop placeholder guide shapes flashing before template loads

seedGuide() ran in onMount via the 'else' branch while template was still null during the async
fetch, creating 5 example shapes (Q1 start/end, part, response, context) that flashed on screen
until the real template + PDF loaded. Only seed the guide for a genuinely-empty template after load.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/pages/exam/setup/ExamTemplateSetupPage.tsx

@@ -211,7 +211,9 @@ const ExamTemplateSetupInner: React.FC = () => {
     setTemplate(detail)
     const editor = editorRef.current
     if (editor) {
-      loadShapes(editor, shapesFromTemplate(detail, pageGeometriesRef.current))
+      const shapes = shapesFromTemplate(detail, pageGeometriesRef.current)
+      loadShapes(editor, shapes)
+      if (!shapes.length) seedGuide(editor)
       bringDomainShapesToFront(editor)
     }
     setDirty(false)
@@ -417,7 +419,14 @@ const ExamTemplateSetupInner: React.FC = () => {
                 editor.store.listen(() => setDirty(true), { scope: 'document' })
                 applyDocViewConstraints(editor, [])
                 editor.resetZoom()
-                if (template) loadShapes(editor, shapesFromTemplate(template, pageGeometriesRef.current)); else seedGuide(editor)
+                // Only seed the example guide for a genuinely-empty template AFTER it has loaded.
+                // (Previously `else seedGuide` fired on mount while `template` was still null during
+                // the async fetch, flashing placeholder shapes before the real shapes/PDF rendered.)
+                if (template) {
+                  const s = shapesFromTemplate(template, pageGeometriesRef.current)
+                  loadShapes(editor, s)
+                  if (!s.length) seedGuide(editor)
+                }
                 bringDomainShapesToFront(editor)
               }}
             />