Implement the lesson columns.

The route is `/lesson/<lesson-slug>`.
The level parameter is not yet taken into account.
The language is not yet taken into account either.

user/plugins/m3-p2/m3-p2.php

@@ -65,8 +65,14 @@ class M3P2Plugin extends Plugin
 //            ]);
 //        }
 
+        $priority = 1; // must be higher than the error plugin's
+
+        $this->enable([
+            'onPageNotFound' => ['perhapsDisplayPolyglotColumns', $priority],
+        ]);
+
         $this->enable([
-            'onPageNotFound' => ['perhapsDisplayPolyglotColumns', 1],
+            'onPageNotFound' => ['perhapsDisplayLessonColumns', $priority],
         ]);
 
 //        if ("" == $current_route) {
@@ -212,4 +218,64 @@ class M3P2Plugin extends Plugin
         $event->page = $page;
         $event->stopPropagation();
     }
+
+    public function perhapsDisplayLessonColumns(Event $event)
+    {
+        /** @var Uri $uri */
+        $uri = $this->grav['uri'];
+//        var_dump($uri); // best doc ftw
+
+        if ("lesson" != $uri->paths(0)) {
+            return; // perhaps not
+        }
+
+        /** @var Pages $pages */
+        $pages = $this->grav['pages'];
+
+        if (count($uri->paths()) < 2) {
+            // user is toying with URI, probably
+            // idea: display an index of the lessons here todo
+            return;
+        }
+
+        $lesson_slug = $uri->paths(1);
+//        var_dump($lesson_slug);
+
+//        $target_pages = $pages->all()->filter(function ($p) use ($lesson_slug) {
+        $target_pages = array_filter($pages->instances(), function (Page $p) use ($lesson_slug) {
+
+            $h = $p->header();
+            if ( ! isset($h->pedagogic_paths) || empty($h->pedagogic_paths)) {
+                return false;
+            }
+
+            foreach ($h->pedagogic_paths as $lesson) {
+                if ($lesson_slug == $lesson['slug']) {
+                    return true; // yay, this page is part of the lesson
+                }
+            }
+
+            return false;
+        });
+
+//        var_dump($target_pages);
+
+        // fixme: sort pages
+
+//        $target_page = $pages->all();
+
+        if (empty($target_pages)) {
+            return;
+        }
+
+        $this->grav['twig']->twig()->addGlobal('target_pages', $target_pages);
+
+        $page = new Page;
+        $page->init(new \SplFileInfo(__DIR__ . '/pages/lesson-columns.md'));
+
+        // here: perhaps add a title and other metadata
+
+        $event->page = $page;
+        $event->stopPropagation();
+    }
 }

user/themes/textbook/templates/lesson-columns.html.twig

+{% extends 'partials/base.html.twig' %}
+
+{# `page` is irrelevant here #}
+{# use `target_pages`, injected by m3-p2 plugin #}
+
+{% block stylesheets %}
+    {% do assets.addCss('theme://css/columns.css') %}
+    {{ parent() }}
+{% endblock %}
+
+{#{% block javascripts %}#}
+    {#{% do assets.addJs('theme://js/syncscroll.js', {group:'bottom'}) %}#}
+    {#{{ parent() }}#}
+{#{% endblock %}#}
+
+
+{% block body %}
+
+    <div class="claude-columns">
+    {% for column in target_pages %}
+        <section id="{{ column.slug }}" class="claude-column" name="language">
+            {{ column.content | raw }}
+            {% include 'partials/contributions-incentive.html.twig' with {page: column} %}
+        </section>
+    {% endfor %}
+    </div>
+
+{% endblock %}
+
+{# Disable the language selector. #}
+{#{% block header_language_selection %}{% endblock %}#}
+{# … disabling the whole section instead is semantically correct but it offsets the navigation. TBD #}
+{#{% block header_language_selection_section %}{% endblock %}#}