refactor(engine): pure cdylib + undo-stack test race fix (M14 R4)

- oakengine is now cdylib-only (no rlib/staticlib consumers anywhere;
  cargo tree verified) — the plugin/external C ABI layer; README and
  docs updated
- cd.yml drops the dylib embedding/re-sign steps (the app no longer
  links it)
- test race root-caused and fixed for good: the global undo stack lock
  is now a re-entrant mutex (parking_lot) shared by every test that
  drives the stack, including the previously unlocked node/render
  families; the render-manager serial-ordering bug (an earlier repro
  test initialized the global manager before the not-initialized test)
  is fixed with a shared SERIAL guard and a manager shutdown
- 5 consecutive parallel runs clean; serial 209/209

docs/build.md

@@ -3,8 +3,11 @@
 This document describes how to build Oak Video Editor from source on Windows, Linux, and macOS.
 
 > **Note (2026):** Oak is now a Rust workspace; `cargo build` at the
-> repository root produces the app, the CLI, the worker and
-> `liboakengine`. The CMake instructions below are kept for historical
+> repository root produces the app, the CLI and the worker
+> (`liboakengine` — the plugin/external-consumer cdylib — is not a
+> default member, M14 R4; build it explicitly with
+> `cargo build -p oakengine`). The CMake instructions below are kept for
+> historical
 > reference only. Rust dependencies are pulled from crates.io; the only
 > native libraries still needed are FFmpeg (see the next section),
 > OpenColorIO (optional; `ocio-sys` builds a stub without it) and a

docs/zh/plans/riir/M14-direct-rlib.md

@@ -45,7 +45,7 @@
 | R1 | 胶水下沉：oakundo 全局栈+通知、oakstorage 会话管理器、oaktask 任务面补齐、oaknode ops 组合函数 | 各模块自带测试绿；facade 改为转发下沉版（行为不变，全量绿） |
 | R2 | oak-cli + oak-worker 切换（小，先蹚路） | 两 crate 零 oakengine 依赖，测试绿 |
 | R3 | app 切换：src/oakui/ffi.rs、host_syms.rs 删除，real.rs 全量改 Rust 调用；AppEngine trait 不动（Mock 保留） | `cargo test`（根 crate）全绿；真机冒烟（导入/播放/编辑/导出） |
-| R4 | oakengine 转纯 cdylib（workspace default-members 移除 rlib 使用点清零）、根 build.rs 链接逻辑删除、CD 去掉 app 内嵌 dylib | CI 绿；dmg 体积显著缩小（不再嵌 37-56MB dylib） |
+| R4 | oakengine 转纯 cdylib（workspace default-members 移除 rlib 使用点清零）、根 build.rs 链接逻辑删除、CD 去掉 app 内嵌 dylib（**已完成，2026-08**：crate-type 已是 cdylib-only、零 Rust 依赖方、cd.yml 去掉 embed 步骤与 `-p oakengine` 预构建） | CI 绿；dmg 体积显著缩小（不再嵌 37-56MB dylib） |
 | R5 | M13 遗留的 CHandle 内部清除逐模块完成（bridge 已删，剩 handle.rs 与模块内 CHandle 传参） | 模块内部无 CHandle 传参；C ABI 导出层（oakengine）独占 CHandle |
 
 依赖：R1→R2→R3→R4；R5 与 R2-R4 可并行。