Ensure backward compatibility for single-player mode

Added explicit tests documenting that createCombatState, drawCards,
playCard, and endTurn all accept the original single-string/no-index
signatures and keep state.player and state.enemy aliases in sync.

src/state.test.js

@@ -226,3 +226,42 @@ describe("endTurn - indexed player", () => {
     expect(next.combat.phase).toBe("player_turn");
   });
 });
+
+describe("backward compat - single string args", () => {
+  test("string args produce state with compat player/enemy aliases", () => {
+    const state = createCombatState("ironclad", "jaw_worm");
+    // compat aliases still work
+    expect(state.player).toBeDefined();
+    expect(state.enemy).toBeDefined();
+    expect(state.player.hp).toBe(11);
+    expect(state.enemy.name).toBe("Jaw Worm");
+    // arrays also populated
+    expect(state.players).toHaveLength(1);
+    expect(state.enemies).toHaveLength(1);
+    expect(state.combat.playerCount).toBe(1);
+  });
+
+  test("drawCards with single count arg still works", () => {
+    const state = createCombatState("ironclad", "jaw_worm");
+    const next = drawCards(state, 3);
+    expect(next.player.hand).toHaveLength(3);
+    expect(next.players[0].hand).toHaveLength(3);
+  });
+
+  test("playCard with just handIndex still works", () => {
+    let state = createCombatState("ironclad", "jaw_worm");
+    state = drawCards(state, 5);
+    const cardIndex = state.player.hand.indexOf("strike_r");
+    const next = playCard(state, cardIndex);
+    expect(next.player.energy).toBe(2);
+    expect(next.players[0].energy).toBe(2);
+  });
+
+  test("endTurn with no playerIndex still works", () => {
+    let state = createCombatState("ironclad", "jaw_worm");
+    state = drawCards(state, 5);
+    const next = endTurn(state);
+    expect(next.combat.phase).toBe("enemy_turn");
+    expect(next.player.hand).toHaveLength(0);
+  });
+});