Characters: seed the current party sheets, add skill expertise

The party sheets in assets/characters/ changed: Darrah_Oakwanderer.pdf is
gone, replaced by Darragh Fermwyr (same halfling ranger, renamed and
updated), Owayn Fermwyr (stout halfling fighter, BJ) and Dunnchadh
(ghostwise halfling druid, Circle of the Blighted, Rick).

- app:character:seed now carries those three sheets, read from the PDFs'
  form fields. Darragh's portrait is renamed to match his new name.
- New skill expertise field (double proficiency bonus), since Darragh's
  Deft Explorer gives him +7 Stealth and the sheet had nowhere to show
  it. Expertise implies proficiency; the sheet marks it with two dots,
  the edit form gets a checkbox list next to skill proficiencies.
- Migration adds skill_expertise, backfilling existing rows with [] so
  the NOT NULL JSON column works on tables that already have characters.

Tests: expertise math on the entity, and a catalog test that every seed
sheet hydrates into a Character that passes validation.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

migrations/Version20261004104738.php

@@ -0,0 +1,32 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DoctrineMigrations;
+
+use Doctrine\DBAL\Schema\Schema;
+use Doctrine\Migrations\AbstractMigration;
+
+/**
+ * Characters: skill expertise (double proficiency bonus).
+ */
+final class Version20261004104738 extends AbstractMigration
+{
+    public function getDescription(): string
+    {
+        return 'Add skill_expertise to character_sheet';
+    }
+
+    public function up(Schema $schema): void
+    {
+        // Existing rows need a valid JSON value before the column can be NOT NULL.
+        $this->addSql('ALTER TABLE character_sheet ADD skill_expertise JSON DEFAULT NULL');
+        $this->addSql("UPDATE character_sheet SET skill_expertise = '[]'");
+        $this->addSql('ALTER TABLE character_sheet MODIFY skill_expertise JSON NOT NULL');
+    }
+
+    public function down(Schema $schema): void
+    {
+        $this->addSql('ALTER TABLE character_sheet DROP skill_expertise');
+    }
+}