Add PF2e encounter difficulty calculation with 5-tier budget system

Implements PF2e encounter difficulty alongside the existing D&D system.
PF2e uses creature level vs party level to derive XP, compares against
5-tier budgets (Trivial/Low/Moderate/Severe/Extreme), and adjusts
thresholds for party size. The indicator shows 4 bars in PF2e mode.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/web/src/__tests__/factories/index.ts

@@ -1,3 +1,4 @@
 export { buildCombatant } from "./build-combatant.js";
 export { buildCreature } from "./build-creature.js";
 export { buildEncounter } from "./build-encounter.js";
+export { buildPf2eCreature } from "./build-pf2e-creature.js";