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/settings-modal.tsx

@@ -36,7 +36,7 @@ export function SettingsModal({ open, onClose }: Readonly<SettingsModalProps>) {
 			<div className="flex flex-col gap-5">
 				<div>
 					<span className="mb-2 block font-medium text-muted-foreground text-sm">
-						Conditions
+						Rules Edition
 					</span>
 					<div className="flex gap-1">
 						{EDITION_OPTIONS.map((opt) => (