Add 2014 DMG encounter difficulty calculation

Support the 2014 DMG encounter difficulty as an alternative to the 5.5e
system behind the existing Rules Edition toggle. The 2014 system uses
Easy/Medium/Hard/Deadly thresholds, an encounter multiplier based on
monster count, and party size adjustment (×0.5–×5 range).

- Extract RulesEdition to its own domain module
- Refactor DifficultyTier to abstract numeric values (0–3)
- Restructure DifficultyResult with thresholds array
- Add 2014 XP thresholds table and encounter multiplier logic
- Wire edition from context into difficulty hooks
- Edition-aware labels in indicator and breakdown panel
- Show multiplier, adjusted XP, and party size note for 2014
- Rename settings label from "Conditions" to "Rules Edition"
- Update spec 008 with issue #23 requirements

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

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

@@ -12,6 +12,7 @@ import {
 } from "../../__tests__/factories/index.js";
 import { AllProviders } from "../../__tests__/test-providers.js";
 import { useDifficultyBreakdown } from "../use-difficulty-breakdown.js";
+import { useRulesEdition } from "../use-rules-edition.js";
 
 beforeAll(() => {
 	Object.defineProperty(globalThis, "matchMedia", {
@@ -292,4 +293,56 @@ describe("useDifficultyBreakdown", () => {
 		expect(breakdown?.enemyCombatants).toHaveLength(1);
 		expect(breakdown?.enemyCombatants[0].combatant.name).toBe("Thug");
 	});
+
+	it("exposes encounterMultiplier and adjustedXp for 5e edition", async () => {
+		const wrapper = makeWrapper({
+			encounter: buildEncounter({
+				combatants: [
+					buildCombatant({
+						id: combatantId("c-1"),
+						name: "Hero",
+						playerCharacterId: pcId1,
+					}),
+					buildCombatant({
+						id: combatantId("c-2"),
+						name: "Goblin",
+						creatureId: goblinCreature.id,
+					}),
+					buildCombatant({
+						id: combatantId("c-3"),
+						name: "Thug",
+						cr: "1",
+					}),
+				],
+			}),
+			playerCharacters: [
+				{ id: pcId1, name: "Hero", ac: 15, maxHp: 30, level: 1 },
+			],
+			creatures: new Map([[goblinCreature.id, goblinCreature]]),
+		});
+
+		const { result: editionResult } = renderHook(() => useRulesEdition(), {
+			wrapper,
+		});
+		editionResult.current.setEdition("5e");
+
+		try {
+			const { result } = renderHook(() => useDifficultyBreakdown(), {
+				wrapper,
+			});
+
+			await waitFor(() => {
+				const breakdown = result.current;
+				expect(breakdown).not.toBeNull();
+				// 2 enemy monsters, 1 PC (<3) → base x1.5, shift up → x2
+				expect(breakdown?.encounterMultiplier).toBe(2);
+				// CR 1/4 (50) + CR 1 (200) = 250, x2 = 500
+				expect(breakdown?.totalMonsterXp).toBe(250);
+				expect(breakdown?.adjustedXp).toBe(500);
+				expect(breakdown?.thresholds).toHaveLength(4);
+			});
+		} finally {
+			editionResult.current.setEdition("5.5e");
+		}
+	});
 });