Fix encounter difficulty tier classification for 5.5e and PF2e

The 2024 DMG and PF2e GM Core define XP budgets as ceilings ("spend as
much of your XP budget as you can without going over"), not floors like
the 2014 DMG thresholds. The tier is now the lowest budget the XP does
not exceed: an encounter over the Moderate budget is High even if below
the High budget. PF2e now also uses the Trivial budget (40 or less),
GM Core remaster party-size adjustments (Low +20), and counts creatures
more than 4 levels below the party as 0 XP. 2014 rules are unchanged.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

apps/web/src/hooks/__tests__/use-difficulty.test.tsx

@@ -341,10 +341,10 @@ describe("useDifficulty", () => {
 		await waitFor(() => {
 			expect(result.current).not.toBeNull();
 			// Level 3 budget: low=150, mod=225, high=400
-			// CR 1/4 = 50 XP -> trivial
+			// CR 1/4 = 50 XP ≤ 150 Low budget -> Low
 			expect(result.current?.thresholds[0].value).toBe(150);
 			expect(result.current?.totalMonsterXp).toBe(50);
-			expect(result.current?.tier).toBe(0);
+			expect(result.current?.tier).toBe(1);
 		});
 	});