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>

src/Form/CharacterType.php

@@ -68,6 +68,14 @@ class CharacterType extends AbstractType
                 'required' => false,
                 'label' => 'Skill proficiencies',
             ])
+            ->add('skillExpertise', ChoiceType::class, [
+                'choices' => array_combine(array_keys(Character::SKILLS), array_keys(Character::SKILLS)),
+                'multiple' => true,
+                'expanded' => true,
+                'required' => false,
+                'label' => 'Skill expertise',
+                'help' => 'Double proficiency bonus (e.g. Deft Explorer, Rogue Expertise)',
+            ])
             // Text blocks
             ->add('otherProficienciesLanguages', TextareaType::class, $block + ['label' => 'Other proficiencies & languages'])
             ->add('equipment', TextareaType::class, $block)