feat(engine): add instant action execution

Implements executeInstant, which applies an instant action's costs and
yields immediately with no queue slot and no duration requirement.

src/engine/__tests__/game.test.ts

@@ -6,6 +6,7 @@ import {
   clearQueue,
   createGameState,
   enqueueAction,
+  executeInstant,
   startAction,
   tickGame,
 } from '../game';
@@ -367,3 +368,37 @@ describe('tickGame()', () => {
     expect(state.activeActionId).toBeNull();
   });
 });
+
+describe('executeInstant()', () => {
+  const instantContent = buildContent({
+    resources: [
+      { id: 'supplies', name: 'Supplies', startAmount: 0 },
+      { id: 'coin', name: 'Coin', startAmount: 5 },
+    ],
+    actions: [
+      {
+        id: 'buy_supply',
+        name: 'Buy supply',
+        kind: 'instant',
+        group: { id: 'buy', label: 'Buy' },
+        costs: [{ resourceId: 'coin', amount: 2 }],
+        yields: [{ resourceId: 'supplies', amount: 1 }],
+      },
+    ],
+  });
+
+  it('applies costs and yields immediately without queueing', () => {
+    const state = createGameState(instantContent);
+    executeInstant(state, instantContent, 'buy_supply');
+    expect(state.resources.coin).toBe(3);
+    expect(state.resources.supplies).toBe(1);
+    expect(state.activeActionId).toBeNull();
+    expect(state.actionQueue).toEqual([]);
+  });
+
+  it('throws when unaffordable', () => {
+    const state = createGameState(instantContent);
+    state.resources.coin = 0;
+    expect(() => executeInstant(state, instantContent, 'buy_supply')).toThrow(/Cannot/);
+  });
+});

src/engine/game.ts

@@ -156,6 +156,23 @@ export function clearQueue(state: GameState): void {
   state.actionQueue.length = 0;
 }
 
+/**
+ * Execute an instant action immediately, deducting costs and granting yields
+ * without occupying a queue slot or requiring a duration.
+ * Throws if the action is not of kind 'instant' or is unavailable.
+ */
+export function executeInstant(state: GameState, content: Content, actionId: string): void {
+  const action = content.actionsById[actionId];
+  if (!action || action.kind !== 'instant') {
+    throw new Error(`Action "${actionId}" is not instant`);
+  }
+  if (!isActionAvailable(state, content, actionId)) {
+    throw new Error(`Cannot perform instant action "${actionId}"`);
+  }
+  deductCosts(state, content, actionId);
+  grantYields(state, content, actionId);
+}
+
 /**
  * Advance the active action by `tickMs`. On completion, grants yields and
  * advances the queue — actions do not auto-repeat when the queue is empty.