docs: 文档审计——修正失效链接与过期结论

按 docs/ 目录逐篇核对，只改与事实/路径不符的部分，不动尚未验证的计划条目：

- `docs/steps/**` 在早前整理时已迁到 `docs/reference/steps/**`，全仓的旧路径
  引用（PLAN.md / README.md / M0-gr2-reader.md 及 libgr2 的头注释）一并改正。
- CLIENT-GAP.md / CLIENT-GAP-FIX.md / BACKLOG.md / MIDREVIEW.md：把已经落地的
  条目从「待办」改为已完成，删掉与代码现状矛盾的描述。
- ANDROID-TESTING.md / CLIENT-PORT.md / CLIENT-ROADMAP.md / GODOT-POC-PLAN.md /
  SHINSOO-WORLD-RENDERING.md / PARITY-GAP.md：同上，另补当前实际的构建/测试入口。
- THIRD-PARTY.md：补齐实际在用的第三方来源与许可说明。

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SJugvEJwz3FK4hw9ti3SRb

docs/reference/README.md

@@ -21,7 +21,7 @@ still authoritative:
 **Relative paths inside these files** (`../libgr2`, `oracle/`, `test/golden/…`,
 `engine/…`, `app/…`) refer to the old xrender-poc layout. The mapping:
 
-| old (xrender-poc) | now (mtgodot-poc) |
+| old (xrender-poc) | now (metin2-client; the repo was created as `mtgodot-poc` and renamed later — the `mtgodot-poc.app` / `.apk` artifact names and the `org.internal.mtgodotpoc` bundle id did **not** change) |
 |---|---|
 | `libgr2/` | `libgr2/` (vendored, unchanged) |
 | `formats/` | `formats/` (vendored) |