test: validate generated HTML API migration lanes

[shczhen]

Summary

• adds a repeatable validation runner for the generated HTML API migration matrix
• records the current real-source validation report for DITA/Oxygen, TypeDoc, Doxygen/Javadoc, iOS doc-generator/appledoc, and Dartdoc
• adds substantive TypeDoc content assertions for TypeDoc API migration drops global type member documentation #715/TypeDoc API migration flattens tables in member documentation #716
• rejects level-7+ generated headings while ignoring fenced code examples
• keeps REST/OpenAPI explicitly out of scope and verifies the expected guidance

Closes #697.

Verification

• bun test scripts/html-to-md-migration.test.ts scripts/validate-html-api-migration.test.ts (13 passed)
• node --check scripts/validate-html-api-migration.mjs
• bunx biome check scripts/validate-html-api-migration.mjs scripts/validate-html-api-migration.test.ts
• bun run html:migration:validate -- --source-root /path/to/shengwang-doc-source/html-docs
• full validation completed against the real legacy source matrix
• temporary generated content passed bun run types:check
• bun run build was not run

Notes

This PR is stacked on #702 so the validation runner exercises all lane implementations without merging them into one review PR.

Set HTML_MIGRATION_SOURCE_ROOT instead of passing --source-root when preferred.

[github-actions]

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[shczhen]

同步 Issue #746 的迁移脚本根因修复与回归测试：DITA 页面 description 不再跨 topic 边界读取首个 member shortdesc。提交 699c50fff；migration 20/20、types:check 通过。仍待 doc team 测试。

[shczhen]

#746 的跨平台回归门禁已同步到本 PR（788d5f832）：

• 真实 DITA 输出校验会拒绝“页面 description 等于首个 API member 描述”。
• 新增已提交 RTC DITA 内容静态门禁，防止脚本正确但生成产物未同步。
• 本 PR 不包含 feat: migrate CN HTML API references #712 的 343 个内容文件更新。
• 验证：迁移与校验测试 24/24、types:check、diff check 全部通过。

PR 保持未合并，待 doc team 测试。

[shczhen]

已在原 PR 为 #730 增加阻断式重复显式锚点验证；与生成侧共用 Markdown code protection scanner，不扫描 fenced/inline/跨行 code 示例。专项 6/6、types:check、Standards/Spec 审查通过。PR 保持未合并，等待 doc team 测试。

[Foesa-yyx]

Validation matrix audit verified cleanly from docs-portal CN-NEWDOC baseline: validation tests passed (7 files / 35 tests), bun run types:check passed, and git diff --check passed. This lane does not generate product content. Closing per audit request.