feat: 驱动按 process.platform 自动选择，非 win32 抛 platform_unsupported

迁移前置项（指南五件小事 #2）：
- RpaEngine.driver 改惰性 getter：win32 惰性创建 WindowsDriver，其他平台访问时
  抛 RuntimeError(platform_unsupported)，import 不再隐式绑定 Windows
- 错误码 platform_unsupported 登记 RUNTIME_ERROR_CODES
- engine-driver.test.ts 覆盖两条路径

packages/rpa-core/src/engine/engine-driver.test.ts

@@ -0,0 +1,22 @@
+import { describe, expect, it } from "vitest";
+import { createEngine } from "./engine";
+import { WindowsDriver } from "../driver/windows/windows-driver";
+
+describe("driver 平台自动选择（迁移指南五件小事 #2）", () => {
+  it("win32 上惰性创建 WindowsDriver 且实例缓存", () => {
+    const engine = createEngine();
+    expect(engine.driver).toBeInstanceOf(WindowsDriver);
+    expect(engine.driver).toBe(engine.driver);
+  });
+
+  it("非 win32 访问 driver 抛 platform_unsupported", () => {
+    const engine = createEngine();
+    const original = process.platform;
+    Object.defineProperty(process, "platform", { value: "darwin", configurable: true });
+    try {
+      expect(() => engine.driver).toThrowError(/暂不支持桌面自动化驱动/);
+    } finally {
+      Object.defineProperty(process, "platform", { value: original, configurable: true });
+    }
+  });
+});

packages/rpa-core/src/engine/engine.ts

@@ -2,6 +2,7 @@ import type { Client } from "basic-ftp";
 import type { Workbook as ExcelJSWorkbook } from "exceljs";
 import { WindowsDriver } from "../driver/windows/windows-driver";
 import type { AutomationDriver } from "../driver/types";
+import { RuntimeError } from "./types";
 
 /** 人为拟真操作的开关与随机参数（desktop 能力的进程级设置）。 */
 export interface ManualOperationSettings {
@@ -41,8 +42,23 @@ export class RpaEngine {
     minMs: 1000,
     maxMs: 3000
   };
-  /** 桌面自动化驱动（任务 3.4 注入点，默认 PowerShell 版 WindowsDriver） */
-  readonly driver: AutomationDriver = new WindowsDriver();
+  /** 桌面自动化驱动：按系统自动选（迁移指南五件小事 #2）。win32 用 PowerShell 版 WindowsDriver；
+   *  其他平台尚未实现，访问时抛 platform_unsupported 明确报错（惰性创建，避免 import 即炸） */
+  private _driver?: AutomationDriver;
+  get driver(): AutomationDriver {
+    if (!this._driver) {
+      if (process.platform === "win32") {
+        this._driver = new WindowsDriver();
+      } else {
+        throw new RuntimeError(
+          "platform_unsupported",
+          `该平台暂不支持桌面自动化驱动（当前：${process.platform}）`,
+          { recommendation: "桌面/键鼠/窗口能力目前仅支持 Windows；macOS/Linux 驱动在迁移指南三端计划中规划" }
+        );
+      }
+    }
+    return this._driver;
+  }
 }
 
 export function createEngine(): RpaEngine {

packages/rpa-core/src/engine/error-codes.ts

@@ -40,6 +40,7 @@ export const RUNTIME_ERROR_CODES = [
   "word_text_not_found",
   "outlook_unavailable",
   // 其他
+  "platform_unsupported",
   "decryption_failed",
   "host_unavailable"
 ] as const;