5e builder: add curated official subclasses (start with Rogue Mastermind)

classes.json ships the SRD subclass + third-party homebrew but is missing most official
PHB/XGE archetypes. New DND5E_SUBCLASSES supplement is merged (deduped by name) ahead of
the on-disk list in the builder, surviving any classes.json regeneration. First entry:
Rogue → Mastermind. (Subclass feature mechanics remain a separate task; this adds the
selectable options.)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/lib/rules/dnd5e/progression.ts

@@ -99,3 +99,16 @@ export function dnd5eAsiLevels(className: string): number[] {
   if (c === 'rogue') return [...base, 10].sort((a, b) => a - b);
   return base;
 }
+
+/**
+ * Curated official 5e subclasses, merged into the builder's class list on top of
+ * whatever the on-disk classes.json carries (which is SRD + third-party homebrew and
+ * is missing most PHB/XGE archetypes). Keyed by class name; deduped by name at merge
+ * time. Descriptions are short "what it plays like" blurbs — subclass *features* are
+ * not yet auto-applied (a separate task), so these add the missing options to pick.
+ */
+export const DND5E_SUBCLASSES: Record<string, { name: string; desc: string }[]> = {
+  Rogue: [
+    { name: 'Mastermind', desc: 'A master of intrigue and tactics: extra tool/language proficiencies, Help as a bonus action at range, and uncanny insight into a creature’s drives.' },
+  ],
+};