Storyline: unfoldable section list per chapter in the sidebar

Each chapter in the right-hand menu gets a caret that unfolds its
numbered sections (e.g. the prologue's 28 scenes); clicking one jumps
straight to it. The chapter you're reading starts unfolded, other
chapters' sections link across (/storyline/overview#section-…), and the
section at the top of the pane is highlighted as you scroll.

- ChapterOutline service parses each rendered chapter for its
  <h2 class="storyline-section"> headings, adds anchor ids and returns the
  outline; the controller renders all chapters once to feed the sidebar.
- storyline-toc Stimulus controller: fold/unfold, in-pane jump, scroll-spy
  (works in the split view and in the small-screen window-scroll fallback).
- Tests for the outline parser.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/Controller/StorylineController.php

@@ -4,6 +4,7 @@ namespace App\Controller;
 
 use App\Form\ChapterNoteType;
 use App\Repository\ChapterNoteRepository;
+use App\Storyline\ChapterOutline;
 use Doctrine\ORM\EntityManagerInterface;
 use Symfony\Bundle\FrameworkBundle\Controller\AbstractController;
 use Symfony\Component\HttpFoundation\Request;
@@ -51,7 +52,7 @@ class StorylineController extends AbstractController
     }
 
     #[Route('/storyline/{slug}', name: 'app_storyline_chapter', requirements: ['slug' => '[a-z0-9-]+'], methods: ['GET', 'POST'])]
-    public function chapter(string $slug, Request $request, ChapterNoteRepository $notes, EntityManagerInterface $em): Response
+    public function chapter(string $slug, Request $request, ChapterNoteRepository $notes, EntityManagerInterface $em, ChapterOutline $outline): Response
     {
         if (!isset(self::CHAPTERS[$slug])) {
             throw $this->createNotFoundException('No such storyline chapter.');
@@ -71,6 +72,18 @@ class StorylineController extends AbstractController
             return $this->redirectToRoute('app_storyline_chapter', ['slug' => $slug]);
         }
 
+        // Render every chapter once: the current one is shown (with anchors),
+        // the rest only feed the sidebar's per-section jump lists.
+        $sections = [];
+        $body = '';
+        foreach (self::CHAPTERS as $s => $c) {
+            $processed = $outline->process($this->renderView($c['template']));
+            $sections[$s] = $processed['sections'];
+            if ($s === $slug) {
+                $body = $processed['html'];
+            }
+        }
+
         $slugs = array_keys(self::CHAPTERS);
         $pos = array_search($slug, $slugs, true);
 
@@ -78,6 +91,8 @@ class StorylineController extends AbstractController
             'chapters' => self::CHAPTERS,
             'slug' => $slug,
             'chapter' => self::CHAPTERS[$slug],
+            'body' => $body,
+            'sections' => $sections,
             'note' => $note,
             'form' => $form,
             'prev' => $pos > 0 ? $slugs[$pos - 1] : null,