Remove demo combatants and allow empty encounters

Empty encounters are now valid (INV-1 updated). New sessions start
with zero combatants instead of pre-populated Aria/Brak/Cael.

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

packages/domain/src/__tests__/advance-turn.test.ts

@@ -169,9 +169,9 @@ describe("advanceTurn", () => {
 	});
 
 	describe("invariants", () => {
-		it("INV-1: createEncounter rejects empty combatant list", () => {
+		it("INV-1: createEncounter accepts empty combatant list", () => {
 			const result = createEncounter([]);
-			expect(isDomainError(result)).toBe(true);
+			expect(isDomainError(result)).toBe(false);
 		});
 
 		it("INV-2: activeIndex always in bounds across all scenarios", () => {

packages/domain/src/types.ts

@@ -38,8 +38,8 @@ function domainError(code: string, message: string): DomainError {
 
 /**
  * Creates a valid Encounter, enforcing INV-1, INV-2, INV-3.
- * - INV-1: At least one combatant required.
- * - INV-2: activeIndex defaults to 0 (always in bounds).
+ * - INV-1: An encounter MAY have zero combatants.
+ * - INV-2: activeIndex defaults to 0 (always in bounds when combatants exist).
  * - INV-3: roundNumber defaults to 1 (positive integer).
  */
 export function createEncounter(
@@ -47,13 +47,10 @@ export function createEncounter(
 	activeIndex = 0,
 	roundNumber = 1,
 ): Encounter | DomainError {
-	if (combatants.length === 0) {
-		return domainError(
-			"invalid-encounter",
-			"An encounter must have at least one combatant",
-		);
-	}
-	if (activeIndex < 0 || activeIndex >= combatants.length) {
+	if (
+		combatants.length > 0 &&
+		(activeIndex < 0 || activeIndex >= combatants.length)
+	) {
 		return domainError(
 			"invalid-encounter",
 			`activeIndex ${activeIndex} out of bounds for ${combatants.length} combatants`,