Atomic addCombatant with optional CombatantInit bag

addCombatant now accepts an optional init parameter for pre-filled stats
(HP, AC, initiative, creatureId, color, icon, playerCharacterId), making
combatant creation a single atomic operation with domain validation.

This eliminates the multi-step store.save() bypass in addFromBestiary and
addFromPlayerCharacter, and removes the CombatantOpts/applyCombatantOpts
helpers. Also extracts shared initiative sort logic into initiative-sort.ts
used by both addCombatant and setInitiative.

Closes #15

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

packages/domain/src/add-combatant.ts

@@ -1,24 +1,94 @@
+import type { CreatureId } from "./creature-types.js";
 import type { DomainEvent } from "./events.js";
-import type { CombatantId, DomainError, Encounter } from "./types.js";
+import { sortByInitiative } from "./initiative-sort.js";
+import type { PlayerCharacterId } from "./player-character-types.js";
+import type {
+	Combatant,
+	CombatantId,
+	DomainError,
+	Encounter,
+} from "./types.js";
+
+export interface CombatantInit {
+	readonly maxHp?: number;
+	readonly ac?: number;
+	readonly initiative?: number;
+	readonly creatureId?: CreatureId;
+	readonly color?: string;
+	readonly icon?: string;
+	readonly playerCharacterId?: PlayerCharacterId;
+}
 
 export interface AddCombatantSuccess {
 	readonly encounter: Encounter;
 	readonly events: DomainEvent[];
 }
 
+function validateInit(init: CombatantInit): DomainError | undefined {
+	if (
+		init.maxHp !== undefined &&
+		(!Number.isInteger(init.maxHp) || init.maxHp < 1)
+	) {
+		return {
+			kind: "domain-error",
+			code: "invalid-max-hp",
+			message: `Max HP must be a positive integer, got ${init.maxHp}`,
+		};
+	}
+	if (init.ac !== undefined && (!Number.isInteger(init.ac) || init.ac < 0)) {
+		return {
+			kind: "domain-error",
+			code: "invalid-ac",
+			message: `AC must be a non-negative integer, got ${init.ac}`,
+		};
+	}
+	if (init.initiative !== undefined && !Number.isInteger(init.initiative)) {
+		return {
+			kind: "domain-error",
+			code: "invalid-initiative",
+			message: `Initiative must be an integer, got ${init.initiative}`,
+		};
+	}
+	return undefined;
+}
+
+function buildCombatant(
+	id: CombatantId,
+	name: string,
+	init?: CombatantInit,
+): Combatant {
+	return {
+		id,
+		name,
+		...(init?.maxHp !== undefined && {
+			maxHp: init.maxHp,
+			currentHp: init.maxHp,
+		}),
+		...(init?.ac !== undefined && { ac: init.ac }),
+		...(init?.initiative !== undefined && { initiative: init.initiative }),
+		...(init?.creatureId !== undefined && { creatureId: init.creatureId }),
+		...(init?.color !== undefined && { color: init.color }),
+		...(init?.icon !== undefined && { icon: init.icon }),
+		...(init?.playerCharacterId !== undefined && {
+			playerCharacterId: init.playerCharacterId,
+		}),
+	};
+}
+
 /**
  * Pure function that adds a combatant to the end of an encounter's list.
  *
  * FR-001: Accepts an Encounter, CombatantId, and name; returns next state + events.
  * FR-002: Appends new combatant to end of combatants list.
  * FR-004: Rejects empty/whitespace-only names with DomainError.
- * FR-005: Does not alter activeIndex or roundNumber.
+ * FR-005: Does not alter activeIndex or roundNumber (unless initiative triggers sort).
  * FR-006: Events returned as values, not dispatched via side effects.
  */
 export function addCombatant(
 	encounter: Encounter,
 	id: CombatantId,
 	name: string,
+	init?: CombatantInit,
 ): AddCombatantSuccess | DomainError {
 	const trimmed = name.trim();
 
@@ -30,12 +100,35 @@ export function addCombatant(
 		};
 	}
 
-	const position = encounter.combatants.length;
+	if (init) {
+		const error = validateInit(init);
+		if (error) return error;
+	}
+
+	const newCombatant = buildCombatant(id, trimmed, init);
+	let combatants: readonly Combatant[] = [
+		...encounter.combatants,
+		newCombatant,
+	];
+	let activeIndex = encounter.activeIndex;
+
+	if (init?.initiative !== undefined) {
+		const activeCombatantId =
+			encounter.combatants.length > 0
+				? encounter.combatants[encounter.activeIndex].id
+				: id;
+
+		const result = sortByInitiative(combatants, activeCombatantId);
+		combatants = result.sorted;
+		activeIndex = result.activeIndex;
+	}
+
+	const position = combatants.findIndex((c) => c.id === id);
 
 	return {
 		encounter: {
-			combatants: [...encounter.combatants, { id, name: trimmed }],
-			activeIndex: encounter.activeIndex,
+			combatants,
+			activeIndex,
 			roundNumber: encounter.roundNumber,
 		},
 		events: [
@@ -44,6 +137,7 @@ export function addCombatant(
 				combatantId: id,
 				name: trimmed,
 				position,
+				init,
 			},
 		],
 	};