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/components/turn-navigation.tsx

@@ -1,9 +1,14 @@
 import { Redo2, StepBack, StepForward, Trash2, Undo2 } from "lucide-react";
 import { useState } from "react";
 import { useEncounterContext } from "../contexts/encounter-context.js";
+import { useRulesEditionContext } from "../contexts/rules-edition-context.js";
 import { useDifficulty } from "../hooks/use-difficulty.js";
 import { DifficultyBreakdownPanel } from "./difficulty-breakdown-panel.js";
-import { DifficultyIndicator } from "./difficulty-indicator.js";
+import {
+	DifficultyIndicator,
+	TIER_LABELS_5_5E,
+	TIER_LABELS_2014,
+} from "./difficulty-indicator.js";
 import { Button } from "./ui/button.js";
 import { ConfirmButton } from "./ui/confirm-button.js";
 
@@ -20,6 +25,8 @@ export function TurnNavigation() {
 	} = useEncounterContext();
 
 	const difficulty = useDifficulty();
+	const { edition } = useRulesEditionContext();
+	const tierLabels = edition === "5e" ? TIER_LABELS_2014 : TIER_LABELS_5_5E;
 	const [showBreakdown, setShowBreakdown] = useState(false);
 	const hasCombatants = encounter.combatants.length > 0;
 	const isAtStart = encounter.roundNumber === 1 && encounter.activeIndex === 0;
@@ -79,6 +86,7 @@ export function TurnNavigation() {
 					<div className="relative mr-1">
 						<DifficultyIndicator
 							result={difficulty}
+							labels={tierLabels}
 							onClick={() => setShowBreakdown((prev) => !prev)}
 						/>
 						{showBreakdown ? (