feat(scripts): PoC 阶段 1 - CLI + Layer00619 BackHall 端到端跑通

- scripts/legacy-layer-to-cocos-prefab-cli.mjs (CLI 入口: <inputXml> [<youlePrefab>] <outputDir>)
- framework-tests/legacy-layer-migration/poc-iconbutton.test.mjs (1 个 node:test 端到端校验 cc.Prefab 头 + group-55 父节点)
- YouleNexus/out/legacy-migration/BackHall_Layer.prefab (生成产物, 437 lines)
- YouleNexus/out/legacy-migration/BackHall_Layer.prefab.diff (vs YouleNexus Layer619_BackOtherGame.prefab, 1244 lines)

累计: 30/30 PASS (29 prior + 1 PoC). Layer00619 (1 Spirit, type=3 ProgressBar, group=55) 是最简 PoC, diff 大是预期 (脚本首个端到端输出 vs 手写 prefab 必然差异大).

Co-Authored-By: Claude Code <noreply@anthropic.com>

cocoscreator_projects/scripts/legacy-layer-to-cocos-prefab-cli.mjs

@@ -0,0 +1,35 @@
+#!/usr/bin/env node
+// CLI 入口: Layer XML → Cocos prefab JSON (+ 对比 diff, 如提供 YouleNexus 现存 prefab).
+//
+// 用法: node legacy-layer-to-cocos-prefab-cli.mjs <input.xml> [<youle-prefab>] <output-dir>
+//
+// 设计要点 (Task 9 brief):
+//   - 只输出到隔离目录 (out/legacy-migration/), 不直接覆盖 YouleNexus 现有 prefab.
+//   - 必填参数: inputXml + outputDir; youlePrefab 可选, 提供且文件存在时生成 .diff.
+//   - diff 命令来自 Git Bash / POSIX 环境 (Windows: Git for Windows 自带 diff).
+import { readLayerXml, emitPrefabJson } from './legacy-layer-to-cocos-prefab.mjs';
+import { writeFileSync, mkdirSync, existsSync } from 'node:fs';
+import { execSync } from 'node:child_process';
+import { join } from 'node:path';
+
+const [, , inputXml, youlePrefab, outputDir] = process.argv;
+if (!inputXml || !outputDir) {
+  console.error('Usage: node legacy-layer-to-cocos-prefab-cli.mjs <input.xml> <youle-prefab-path?> <output-dir>');
+  process.exit(1);
+}
+
+const layer = readLayerXml(inputXml);
+const prefabJson = emitPrefabJson(layer);
+
+if (!existsSync(outputDir)) mkdirSync(outputDir, { recursive: true });
+const outputPath = join(outputDir, `${layer.layerName}.prefab`);
+writeFileSync(outputPath, JSON.stringify(prefabJson, null, 2));
+console.log(`Generated: ${outputPath}`);
+
+if (youlePrefab && existsSync(youlePrefab)) {
+  const diffPath = `${outputPath}.diff`;
+  // `|| true` 确保 diff 找到差异时返回非零退出码不致 CLI 崩溃.
+  const diff = execSync(`diff -u "${youlePrefab}" "${outputPath}" || true`, { encoding: 'utf8' });
+  writeFileSync(diffPath, diff);
+  console.log(`Diff: ${diffPath} (${diff.split('\n').length} lines)`);
+}