test(01-02): RED — recompute-derived-stats spec (Pitfall #8/#9)

Task 4 RED phase: 5 failing tests covering pure recompute pipeline.
- hpMax = ancestryHP + (classHP + conMod) × newLevel
- boost-cap-at-18 for CON boost (Pitfall #8)
- proficiencyChanges from ClassProgression applied (fortitude EXPERT)
- Pitfall #9: result MUST NOT contain hpCurrent or hpTemp
- level passthrough

Verified failure: module './recompute-derived-stats' not found.

server/src/modules/leveling/lib/recompute-derived-stats.spec.ts

@@ -0,0 +1,89 @@
+import { recomputeDerivedStats } from './recompute-derived-stats';
+import type { CharacterContext, ClassProgressionRow, WizardChoices, Proficiency } from './types';
+
+type RecomputeInput = CharacterContext & {
+  ancestryHp: number;
+  classHp: number;
+  armorAc: number;
+  armorProficiency: Proficiency;
+  dexCap: number;
+};
+
+function baseCharacter(overrides: Partial<RecomputeInput> = {}): RecomputeInput {
+  return {
+    level: 4,
+    className: 'Fighter',
+    ancestryName: 'Human',
+    heritageName: undefined,
+    abilities: { STR: 18, DEX: 14, CON: 16, INT: 10, WIS: 12, CHA: 10 },
+    skills: {},
+    feats: new Set(),
+    ancestryHp: 8,
+    classHp: 10,
+    armorAc: 4,
+    armorProficiency: 'EXPERT',
+    dexCap: 2,
+    ...overrides,
+  };
+}
+
+function emptyProgression(level: number): ClassProgressionRow {
+  return {
+    className: 'Fighter',
+    level,
+    grants: [],
+    proficiencyChanges: {},
+  };
+}
+
+describe('recomputeDerivedStats — hpMax', () => {
+  it('computes hpMax = ancestryHP + (classHP + conMod) × newLevel for L4 → L5 Fighter, CON 16', () => {
+    // CON 16 → mod +3; classHP 10 + 3 = 13 per level; ancestryHP 8; L5: 8 + 13×5 = 73
+    const ch = baseCharacter();
+    const choices: WizardChoices = {};
+    const result = recomputeDerivedStats(ch, choices, emptyProgression(5));
+    expect(result.hpMax).toBe(73);
+  });
+
+  it('respects boost-cap-at-18 when CON is boosted from 18', () => {
+    // CON starts at 18, boost target includes CON → new CON = 19 (not 20). conMod = +4.
+    // L5: 8 + (10+4)×5 = 78
+    const ch = baseCharacter({
+      abilities: { STR: 16, DEX: 14, CON: 18, INT: 10, WIS: 12, CHA: 10 },
+    });
+    const choices: WizardChoices = { boostTargets: ['CON', 'STR', 'DEX', 'INT'] };
+    const result = recomputeDerivedStats(ch, choices, emptyProgression(5));
+    expect(result.hpMax).toBe(78);
+  });
+});
+
+describe('recomputeDerivedStats — proficiencyChanges from ClassProgression', () => {
+  it('applies proficiencyChanges from ClassProgression at the new level (fortitude EXPERT)', () => {
+    // Fighter L4 → L5 with progression bumping fort to EXPERT.
+    // CON mod +3, level 5; fort = conMod + (level + 4) = 3 + 9 = 12
+    const ch = baseCharacter();
+    const progression: ClassProgressionRow = {
+      ...emptyProgression(5),
+      proficiencyChanges: { fortitude: 'EXPERT' },
+    };
+    const result = recomputeDerivedStats(ch, {}, progression);
+    expect(result.fortitude).toBe(12);
+  });
+});
+
+describe('recomputeDerivedStats — Pitfall #9 (hpCurrent must not be in output)', () => {
+  it('does NOT include hpCurrent in the result object', () => {
+    const ch = baseCharacter();
+    const result = recomputeDerivedStats(ch, {}, emptyProgression(5));
+    expect(result).not.toHaveProperty('hpCurrent');
+    expect(result).not.toHaveProperty('hpTemp');
+  });
+});
+
+describe('recomputeDerivedStats — level passthrough', () => {
+  it('returns the new level in the output', () => {
+    const ch = baseCharacter();
+    const result = recomputeDerivedStats(ch, {}, emptyProgression(5));
+    expect(result.level).toBe(5);
+  });
+});