fix: 修复构建脚本跨平台 import specifier 路径规范化

- toImportSpecifier() 使用 replaceAll 替代 split(sep).join，明确 ESM import specifier 语义
- 增加 relativePath 可选参数支持测试注入 Windows relative 语义
- 重写 Windows 路径测试，使用 node:path.win32 显式模拟而非依赖当前平台 sep
- 更新 DEVELOPMENT.md 记录构建 code generation 约定
- 同步 static-asset-embedding 和 windows-test-compat spec 新增要求

tests/scripts/build-common.test.ts

@@ -1,7 +1,7 @@
 import { afterEach, beforeEach, describe, expect, test } from "bun:test";
 import { mkdir, rm, writeFile } from "node:fs/promises";
 import { tmpdir } from "node:os";
-import { join, sep } from "node:path";
+import { join, win32 } from "node:path";
 
 import { scanDir, toImportSpecifier } from "../../scripts/build-common";
 
@@ -65,10 +65,8 @@ describe("toImportSpecifier", () => {
   });
 
   test("Windows 路径分隔符转换为正斜杠", () => {
-    const fromDir = ["C:", "project", ".build"].join(sep);
-    const targetPath = ["C:", "project", "dist", "web", "assets", "app.js"].join(sep);
-    const result = toImportSpecifier(fromDir, targetPath);
+    const result = toImportSpecifier("C:\\project\\.build", "C:\\project\\dist\\web\\assets\\app.js", win32.relative);
     expect(result).toBe("../dist/web/assets/app.js");
-    expect(result).not.toContain(sep);
+    expect(result).not.toContain("\\");
   });
 });