Storyline: add the Prologue chapter, extend the layout for sessions

Chapters now carry an eyebrow ("Overview", "Prologue", ...) and a short
tag for the list/sidebar badge, instead of a bare running number, so
non-numbered entries read correctly.

New chapter: "Prologue - The Long Road to Brackenford", the session
outline formatted as a runnable document -- an at-a-glance metadata band
(levels, duration, locations, hook), the in-world courier notice as a
posted handout, numbered scenes, GM callouts for the "ask the players"
prompts, label/body rows for the skill-check clues and travel-day
prompts, a three-stage encounter block, dramatic beat lines, and a
before/after panel for the legend the party's own retelling becomes.
Cross-links the Hearthburrow to Enemies and the closing line to the
overview's "Five and the Five".

Tests: render every chapter partial headless (no HTTP/DB) so a broken
variable or path() fails loudly, check each registered chapter has its
template file, and assert the /storyline routes sit behind the login gate.

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

tests/Controller/StorylineControllerTest.php

@@ -0,0 +1,26 @@
+<?php
+
+namespace App\Tests\Controller;
+
+use Symfony\Bundle\FrameworkBundle\Test\WebTestCase;
+
+class StorylineControllerTest extends WebTestCase
+{
+    public function testStorylineRequiresLogin(): void
+    {
+        $client = static::createClient();
+        $client->request('GET', '/storyline');
+
+        self::assertResponseRedirects('/login');
+    }
+
+    public function testUnknownChapterAlsoGoesThroughTheLoginGate(): void
+    {
+        $client = static::createClient();
+        $client->request('GET', '/storyline/does-not-exist');
+
+        // Behind IS_AUTHENTICATED_FULLY, so an anonymous hit redirects rather
+        // than 404s; the 404 path itself is covered in the render test below.
+        self::assertResponseRedirects('/login');
+    }
+}