feat(01-02): implement skill-increase-cap (LVL-06)

Task 2 GREEN phase. PF2e skill-increase cap rule:
- UNTRAINED → TRAINED at any level
- TRAINED → EXPERT requires L3+
- EXPERT → MASTER requires L7+
- MASTER → LEGENDARY requires L15+

Pure function, no NestJS, no Prisma. 9 tests passing.

server/src/modules/leveling/lib/skill-increase-cap.ts

@@ -0,0 +1,31 @@
+import type { Proficiency } from './types';
+
+/** Levels at which a skill-increase step occurs (PF2e CRB). */
+export const SKILL_INCREASE_LEVELS: readonly number[] = [3, 5, 7, 9, 11, 13, 15, 17, 19];
+
+/**
+ * Minimum character level required to advance a skill from `currentRank`.
+ * UNTRAINED → TRAINED is unrestricted (any level >= 1 with a skill-increase step).
+ * LEGENDARY is `null` because it is the maximum rank — no further increases possible.
+ */
+const SKILL_RANK_LEVEL_REQUIREMENTS: Record<Proficiency, number | null> = {
+  UNTRAINED: 1, // → TRAINED
+  TRAINED: 3, // → EXPERT (PF2e CRB)
+  EXPERT: 7, // → MASTER
+  MASTER: 15, // → LEGENDARY
+  LEGENDARY: null, // already maxed
+};
+
+/**
+ * PF2e skill-increase cap rule (LVL-06).
+ * Returns true if a character at `characterLevel` may advance a skill that is
+ * currently at `currentRank` to the next rank.
+ */
+export function canIncreaseSkill(
+  currentRank: Proficiency,
+  characterLevel: number,
+): boolean {
+  const required = SKILL_RANK_LEVEL_REQUIREMENTS[currentRank];
+  if (required === null) return false;
+  return characterLevel >= required;
+}