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/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.