Add combatant side assignment for encounter difficulty

Combatants can now be assigned to party or enemy side via a toggle
in the difficulty breakdown panel. Party-side NPCs subtract their XP
from the encounter total, letting allied NPCs reduce difficulty.
PCs default to party, non-PCs to enemy — users who don't use sides
see no change. Side persists across reload and export/import.

Closes #22

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

apps/web/src/hooks/use-difficulty-breakdown.ts

@@ -9,6 +9,7 @@ import { useMemo } from "react";
 import { useBestiaryContext } from "../contexts/bestiary-context.js";
 import { useEncounterContext } from "../contexts/encounter-context.js";
 import { usePlayerCharactersContext } from "../contexts/player-characters-context.js";
+import { resolveSide } from "./use-difficulty.js";
 
 export interface BreakdownCombatant {
 	readonly combatant: Combatant;
@@ -16,6 +17,8 @@ export interface BreakdownCombatant {
 	readonly xp: number | null;
 	readonly source: string | null;
 	readonly editable: boolean;
+	readonly side: "party" | "enemy";
+	readonly level: number | undefined;
 }
 
 interface DifficultyBreakdown {
@@ -27,7 +30,8 @@ interface DifficultyBreakdown {
 		readonly high: number;
 	};
 	readonly pcCount: number;
-	readonly combatants: readonly BreakdownCombatant[];
+	readonly partyCombatants: readonly BreakdownCombatant[];
+	readonly enemyCombatants: readonly BreakdownCombatant[];
 }
 
 export function useDifficultyBreakdown(): DifficultyBreakdown | null {
@@ -36,105 +40,129 @@ export function useDifficultyBreakdown(): DifficultyBreakdown | null {
 	const { getCreature } = useBestiaryContext();
 
 	return useMemo(() => {
-		const partyLevels = derivePartyLevels(encounter.combatants, characters);
-		const { entries, crs } = classifyCombatants(
-			encounter.combatants,
-			getCreature,
+		const { partyCombatants, enemyCombatants, descriptors, pcCount } =
+			classifyCombatants(encounter.combatants, characters, getCreature);
+
+		const hasPartyLevel = descriptors.some(
+			(d) => d.side === "party" && d.level !== undefined,
 		);
+		const hasCr = descriptors.some((d) => d.cr !== undefined);
 
-		if (partyLevels.length === 0 || crs.length === 0) {
-			return null;
-		}
+		if (!hasPartyLevel || !hasCr) return null;
 
-		const result = calculateEncounterDifficulty(partyLevels, crs);
+		const result = calculateEncounterDifficulty(descriptors);
 
 		return {
 			...result,
-			pcCount: partyLevels.length,
-			combatants: entries,
+			pcCount,
+			partyCombatants,
+			enemyCombatants,
 		};
 	}, [encounter.combatants, characters, getCreature]);
 }
 
-function classifyBestiaryCombatant(
+type CreatureInfo = {
+	cr: string;
+	source: string;
+	sourceDisplayName: string;
+};
+
+function buildBreakdownEntry(
 	c: Combatant,
-	getCreature: (
-		id: CreatureId,
-	) => { cr: string; source: string; sourceDisplayName: string } | undefined,
-): { entry: BreakdownCombatant; cr: string | null } {
-	const creature = c.creatureId ? getCreature(c.creatureId) : undefined;
-	if (creature) {
+	side: "party" | "enemy",
+	level: number | undefined,
+	creature: CreatureInfo | undefined,
+): BreakdownCombatant {
+	if (c.playerCharacterId) {
 		return {
-			entry: {
-				combatant: c,
-				cr: creature.cr,
-				xp: crToXp(creature.cr),
-				source: creature.sourceDisplayName ?? creature.source,
-				editable: false,
-			},
-			cr: creature.cr,
-		};
-	}
-	return {
-		entry: {
 			combatant: c,
 			cr: null,
 			xp: null,
 			source: null,
 			editable: false,
-		},
+			side,
+			level,
+		};
+	}
+	if (creature) {
+		return {
+			combatant: c,
+			cr: creature.cr,
+			xp: crToXp(creature.cr),
+			source: creature.sourceDisplayName ?? creature.source,
+			editable: false,
+			side,
+			level: undefined,
+		};
+	}
+	if (c.cr) {
+		return {
+			combatant: c,
+			cr: c.cr,
+			xp: crToXp(c.cr),
+			source: null,
+			editable: true,
+			side,
+			level: undefined,
+		};
+	}
+	return {
+		combatant: c,
 		cr: null,
+		xp: null,
+		source: null,
+		editable: !c.creatureId,
+		side,
+		level: undefined,
 	};
 }
 
+function resolveLevel(
+	c: Combatant,
+	characters: readonly PlayerCharacter[],
+): number | undefined {
+	if (!c.playerCharacterId) return undefined;
+	return characters.find((p) => p.id === c.playerCharacterId)?.level;
+}
+
+function resolveCr(
+	c: Combatant,
+	getCreature: (id: CreatureId) => CreatureInfo | undefined,
+): { cr: string | null; creature: CreatureInfo | undefined } {
+	const creature = c.creatureId ? getCreature(c.creatureId) : undefined;
+	const cr = creature ? creature.cr : (c.cr ?? null);
+	return { cr, creature };
+}
+
 function classifyCombatants(
-	combatants: readonly Combatant[],
-	getCreature: (
-		id: CreatureId,
-	) => { cr: string; source: string; sourceDisplayName: string } | undefined,
-): { entries: BreakdownCombatant[]; crs: string[] } {
-	const entries: BreakdownCombatant[] = [];
-	const crs: string[] = [];
-
-	for (const c of combatants) {
-		if (c.playerCharacterId) continue;
-
-		if (c.creatureId) {
-			const { entry, cr } = classifyBestiaryCombatant(c, getCreature);
-			entries.push(entry);
-			if (cr) crs.push(cr);
-		} else if (c.cr) {
-			crs.push(c.cr);
-			entries.push({
-				combatant: c,
-				cr: c.cr,
-				xp: crToXp(c.cr),
-				source: null,
-				editable: true,
-			});
-		} else {
-			entries.push({
-				combatant: c,
-				cr: null,
-				xp: null,
-				source: null,
-				editable: true,
-			});
-		}
-	}
-
-	return { entries, crs };
-}
-
-function derivePartyLevels(
 	combatants: readonly Combatant[],
 	characters: readonly PlayerCharacter[],
-): number[] {
-	const levels: number[] = [];
+	getCreature: (id: CreatureId) => CreatureInfo | undefined,
+) {
+	const partyCombatants: BreakdownCombatant[] = [];
+	const enemyCombatants: BreakdownCombatant[] = [];
+	const descriptors: {
+		level?: number;
+		cr?: string;
+		side: "party" | "enemy";
+	}[] = [];
+	let pcCount = 0;
+
 	for (const c of combatants) {
-		if (!c.playerCharacterId) continue;
-		const pc = characters.find((p) => p.id === c.playerCharacterId);
-		if (pc?.level !== undefined) levels.push(pc.level);
+		const side = resolveSide(c);
+		const level = resolveLevel(c, characters);
+		if (level !== undefined) pcCount++;
+
+		const { cr, creature } = resolveCr(c, getCreature);
+
+		if (level !== undefined || cr != null) {
+			descriptors.push({ level, cr: cr ?? undefined, side });
+		}
+
+		const entry = buildBreakdownEntry(c, side, level, creature);
+		const target = side === "party" ? partyCombatants : enemyCombatants;
+		target.push(entry);
 	}
-	return levels;
+
+	return { partyCombatants, enemyCombatants, descriptors, pcCount };
 }