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/build-pf2e-creature.ts

@@ -0,0 +1,28 @@
+import type { Pf2eCreature } from "@initiative/domain";
+import { creatureId } from "@initiative/domain";
+
+let counter = 0;
+
+export function buildPf2eCreature(
+	overrides?: Partial<Pf2eCreature>,
+): Pf2eCreature {
+	const id = ++counter;
+	return {
+		system: "pf2e",
+		id: creatureId(`pf2e-creature-${id}`),
+		name: `PF2e Creature ${id}`,
+		source: "crb",
+		sourceDisplayName: "Core Rulebook",
+		level: 1,
+		traits: ["humanoid"],
+		perception: 5,
+		abilityMods: { str: 2, dex: 1, con: 2, int: 0, wis: 1, cha: -1 },
+		ac: 15,
+		saveFort: 7,
+		saveRef: 4,
+		saveWill: 5,
+		hp: 20,
+		speed: "25 ft.",
+		...overrides,
+	};
+}