docs: 改掉 step 2 里「每道练习都能编译并运行」这句错话

原话把 `mcpp test` 说成「验证每道练习都能编译并运行」，正好说反了：练习发
下来就是没做完的，`std:endl`、`D2X_YOUR_ANSWER` 这类东西**必然编不过**，
pristine 状态下 0 passed 才是对的（e2e.sh 的断言 1 就是在守这条）。

顺带澄清一个容易误会的点：`solutions/` 不是工作区成员（mcpp.toml 的
members 里没有它），`mcpp test` 从头到尾不碰它。要它变绿得先覆盖到练习
文件上——那是 CI 的动作，不是学习者跑一条命令就会发生的事。

改成说清楚它到底验了什么：工具链解析成功、d2x 库编译通过、运行器逐题都
跑到了；练习本身红着是设计如此。README 三语 + book 两语 + chapter_1 两语
共 7 处同步。

Author: Sunrisepeak

README.md

@@ -77,13 +77,14 @@ xlings install d2x mcpp -y   # d2x: exercise framework CLI | mcpp: C++ build & t
 ```bash
 d2x install d2mcpp           # download the course, toolchain auto-configured
 cd d2mcpp
-mcpp test                    # verify the environment: every exercise compiles and runs
+mcpp test                    # environment self-check — prints your progress table
 ```
 
-> The exercises **are** the tests. On a fresh checkout `mcpp test` reports them all as
-> failing — that is the expected starting point, and it already proves your toolchain
-> works. Drop the reference answers from `solutions/` in and the very same command goes
-> all green.
+> The exercises **are** the tests, and they ship unfinished — so a fresh `mcpp test` is
+> *supposed* to come out all red. What it verifies is everything around them: the
+> toolchain resolved, the `d2x` library built, and the runner reached every exercise.
+> Copy `solutions/` over the exercise files and the very same command goes all green
+> (that is exactly what CI does).
 
 <details>
   <summary>optional - start from the git source instead</summary>

README.zh.hant.md

@@ -77,11 +77,12 @@ xlings install d2x mcpp -y   # d2x: 練習框架 CLI | mcpp: C++ 建置與測試
 ```bash
 d2x install d2mcpp           # 下載課程,環境自動配置
 cd d2mcpp
-mcpp test                    # 驗證環境: 每道練習都能編譯並執行
+mcpp test                    # 環境自檢 —— 印出你的進度表
 ```
 
-> 練習**就是**測試。剛拿到課程時 `mcpp test` 會報告全部未通過 —— 這正是練習的起點,
-> 同時也說明工具鏈已經跑通了。把 `solutions/` 裡的參考答案覆蓋進去,同一條指令就會全綠。
+> 練習**就是**測試, 而且發下來時是沒做完的 —— 所以剛拿到課程時 `mcpp test` **本來就該全紅**。
+> 它驗的是練習之外的一切: 工具鏈解析成功、`d2x` 函式庫編譯通過、測試執行器逐題都跑到了。
+> 把 `solutions/` 覆蓋到練習檔案上, 同一條指令就會全綠(CI 做的正是這件事)。
 
 <details>
   <summary>可選 - 改用 clone 專案原始碼的方式</summary>

README.zh.md

@@ -77,11 +77,12 @@ xlings install d2x mcpp -y   # d2x: 练习框架 CLI | mcpp: C++ 构建与测试
 ```bash
 d2x install d2mcpp           # 下载课程,环境自动配置
 cd d2mcpp
-mcpp test                    # 验证环境: 每道练习都能编译并运行
+mcpp test                    # 环境自检 —— 打印你的进度表
 ```
 
-> 练习**就是**测试。刚拿到课程时 `mcpp test` 会报告全部未通过 —— 这正是练习的起点,
-> 同时也说明工具链已经跑通了。把 `solutions/` 里的参考答案覆盖进去,同一条命令就会全绿。
+> 练习**就是**测试, 而且发下来时是没做完的 —— 所以刚拿到课程时 `mcpp test` **本来就该全红**。
+> 它验的是练习之外的一切: 工具链解析成功、`d2x` 库编译通过、测试运行器逐题都跑到了。
+> 把 `solutions/` 覆盖到练习文件上, 同一条命令就会全绿(CI 做的正是这件事)。
 
 <details>
   <summary>可选 - 改用 clone 项目源码的方式</summary>

book/en/src/README.md

@@ -73,13 +73,14 @@ xlings install d2x mcpp -y   # d2x: exercise framework CLI | mcpp: C++ build & t
 ```bash
 d2x install d2mcpp           # download the course, toolchain auto-configured
 cd d2mcpp
-mcpp test                    # verify the environment: every exercise compiles and runs
+mcpp test                    # environment self-check — prints your progress table
 ```
 
-> The exercises **are** the tests. On a fresh checkout `mcpp test` reports them all as
-> failing — that is the expected starting point, and it already proves your toolchain
-> works. Drop the reference answers from `solutions/` in and the very same command goes
-> all green.
+> The exercises **are** the tests, and they ship unfinished — so a fresh `mcpp test` is
+> *supposed* to come out all red. What it verifies is everything around them: the
+> toolchain resolved, the `d2x` library built, and the runner reached every exercise.
+> Copy `solutions/` over the exercise files and the very same command goes all green
+> (that is exactly what CI does).
 
 <details>
   <summary>optional - start from the git source instead</summary>

book/en/src/base/chapter_1.md

@@ -57,7 +57,7 @@ cd d2mcpp
 mcpp test
 ```
 
-> The exercises **are** the tests. On a fresh checkout this reports them all as failing — that is the expected starting point, and it already proves the compile-and-run path works. Drop the reference answers from `solutions/` in and the very same command goes all green.
+> The exercises **are** the tests, and they ship unfinished — so this is *supposed* to come out all red. What it verifies is everything around them: the toolchain resolved, the `d2x` library built, and the runner reached every exercise. Note that `solutions/` is not a workspace member, so `mcpp test` never runs it; copy it over the exercise files and only then does the same command go all green (that is exactly what CI does).
 
 ### Local E-book
 

book/src/README.md

@@ -73,11 +73,12 @@ xlings install d2x mcpp -y   # d2x: 练习框架 CLI | mcpp: C++ 构建与测试
 ```bash
 d2x install d2mcpp           # 下载课程,环境自动配置
 cd d2mcpp
-mcpp test                    # 验证环境: 每道练习都能编译并运行
+mcpp test                    # 环境自检 —— 打印你的进度表
 ```
 
-> 练习**就是**测试。刚拿到课程时 `mcpp test` 会报告全部未通过 —— 这正是练习的起点,
-> 同时也说明工具链已经跑通了。把 `solutions/` 里的参考答案覆盖进去,同一条命令就会全绿。
+> 练习**就是**测试, 而且发下来时是没做完的 —— 所以刚拿到课程时 `mcpp test` **本来就该全红**。
+> 它验的是练习之外的一切: 工具链解析成功、`d2x` 库编译通过、测试运行器逐题都跑到了。
+> 把 `solutions/` 覆盖到练习文件上, 同一条命令就会全绿(CI 做的正是这件事)。
 
 <details>
   <summary>可选 - 改用 clone 项目源码的方式</summary>

book/src/base/chapter_1.md

@@ -57,7 +57,7 @@ cd d2mcpp
 mcpp test
 ```
 
-> 练习**就是**测试。刚拿到课程时这里会报告全部未通过 —— 这正是练习的起点, 同时也说明编译运行这条链路已经跑通了。把 `solutions/` 里的参考答案覆盖进去, 同一条命令就会全绿。
+> 练习**就是**测试, 而且发下来时是没做完的 —— 所以这里**本来就该全红**。它验的是练习之外的一切: 工具链解析成功、`d2x` 库编译通过、测试运行器逐题都跑到了。注意 `solutions/` 不是工作区成员, `mcpp test` 不会去跑它; 把它覆盖到练习文件上, 同一条命令才会全绿(CI 做的正是这件事)。
 
 ### 本地电子书