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>

packages/domain/src/index.ts

@@ -64,6 +64,8 @@ export {
 	type DifficultyResult,
 	type DifficultyThreshold,
 	type DifficultyTier,
+	derivePartyLevel,
+	pf2eCreatureXp,
 	VALID_CR_VALUES,
 } from "./encounter-difficulty.js";
 export type {