Implement the 016-combatant-ac feature that adds an optional Armor Class field to combatants with shield icon display and inline editing in the encounter tracker

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/domain/src/set-ac.ts

@@ -0,0 +1,56 @@
+import type { DomainEvent } from "./events.js";
+import type { CombatantId, DomainError, Encounter } from "./types.js";
+
+export interface SetAcSuccess {
+	readonly encounter: Encounter;
+	readonly events: DomainEvent[];
+}
+
+export function setAc(
+	encounter: Encounter,
+	combatantId: CombatantId,
+	value: number | undefined,
+): SetAcSuccess | DomainError {
+	const targetIdx = encounter.combatants.findIndex((c) => c.id === combatantId);
+
+	if (targetIdx === -1) {
+		return {
+			kind: "domain-error",
+			code: "combatant-not-found",
+			message: `No combatant found with ID "${combatantId}"`,
+		};
+	}
+
+	if (value !== undefined) {
+		if (!Number.isInteger(value) || value < 0) {
+			return {
+				kind: "domain-error",
+				code: "invalid-ac",
+				message: `AC must be a non-negative integer, got ${value}`,
+			};
+		}
+	}
+
+	const target = encounter.combatants[targetIdx];
+	const previousAc = target.ac;
+
+	const updatedCombatants = encounter.combatants.map((c) =>
+		c.id === combatantId ? { ...c, ac: value } : c,
+	);
+
+	return {
+		encounter: {
+			combatants: updatedCombatants,
+			activeIndex: encounter.activeIndex,
+			roundNumber: encounter.roundNumber,
+		},
+		events: [
+			{
+				type: "AcSet",
+				combatantId,
+				previousAc,
+				newAc: value,
+			},
+		],
+	};
+}