feat: discipline UI improvements - stat labels, prerequisites, mana type tab

- Add player-friendly label field to statBonus in DisciplineDefinition
- Show prerequisite requirements on locked discipline cards
- Disable activate button for locked disciplines
- Restructure elemental attunement into dedicated 'Mana Types' tab
- Add checkDisciplinePrerequisites utility function
- Update store to enforce prerequisite checking on activation
- Split discipline-prerequisites tests into separate file

src/lib/game/utils/discipline-math.ts

@@ -87,6 +87,34 @@ export function canProceedDiscipline(
   return element && element.current >= drain;
 }
 
+/**
+ * Check if a discipline's prerequisites are met.
+ * Returns { canProceed: boolean, missingPrereqs: string[] }
+ * where missingPrereqs is a list of prerequisite discipline names that are not yet unlocked.
+ */
+export function checkDisciplinePrerequisites(
+  discipline: DisciplineDefinition,
+  allDisciplines: Record<string, DisciplineState>,
+  allDefinitions: DisciplineDefinition[],
+): { canProceed: boolean; missingPrereqs: string[] } {
+  if (!discipline.requires || discipline.requires.length === 0) {
+    return { canProceed: true, missingPrereqs: [] };
+  }
+
+  const missingPrereqs: string[] = [];
+
+  for (const reqId of discipline.requires) {
+    const reqState = allDisciplines[reqId];
+    // A prerequisite is met if the discipline has XP > 0 (has been practiced)
+    if (!reqState || reqState.xp <= 0) {
+      const reqDef = allDefinitions.find((d) => d.id === reqId);
+      missingPrereqs.push(reqDef?.name ?? reqId);
+    }
+  }
+
+  return { canProceed: missingPrereqs.length === 0, missingPrereqs };
+}
+
 /**
  * Get unlocked perks for a discipline
  */