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>

tests/Storyline/ChapterOutlineTest.php

@@ -0,0 +1,34 @@
+<?php
+
+namespace App\Tests\Storyline;
+
+use App\Storyline\ChapterOutline;
+use PHPUnit\Framework\TestCase;
+
+class ChapterOutlineTest extends TestCase
+{
+    public function testFindsSectionsAndAddsAnchors(): void
+    {
+        $html = '<p>intro</p>'
+            .'<h2 class="storyline-section"><span>21</span> Finding the Hearthburrow</h2><p>x</p>'
+            .'<h2 class="storyline-section"><span>&plus;</span> Appendix &mdash; Timing</h2>';
+
+        $out = (new ChapterOutline())->process($html);
+
+        self::assertSame('21', $out['sections'][0]['num']);
+        self::assertSame('Finding the Hearthburrow', $out['sections'][0]['title']);
+        self::assertSame('section-1-finding-the-hearthburrow', $out['sections'][0]['id']);
+        self::assertSame('+', $out['sections'][1]['num']);
+        self::assertSame('Appendix — Timing', $out['sections'][1]['title']);
+        self::assertStringContainsString('<h2 class="storyline-section" id="section-1-finding-the-hearthburrow">', $out['html']);
+        self::assertStringContainsString('<p>intro</p>', $out['html']);
+    }
+
+    public function testChapterWithoutSectionsIsUntouched(): void
+    {
+        $out = (new ChapterOutline())->process('<p>plain</p>');
+
+        self::assertSame([], $out['sections']);
+        self::assertSame('<p>plain</p>', $out['html']);
+    }
+}