feat: std::random_device works, and it took four layers to make it so

使用者报 `_LIBCPP_HAS_RANDOM_DEVICE 0` 让 5 个 TU 编不过,本地改成 1 即修好。
⚠️ 那个 0 不是保守设置,是当时事实的准确记录 —— 改成 1 只让编译期通过,
链接会缺符号。现在 openkal 有了随机源接口,这四层可以逐一拆掉。

**① 宏改为 1** —— 只在 `generic`;`freestanding` 保持 0,因为裸机后端
只有板子有源时才提供 `openkal.random`,而 6.1 条在链接期报告缺席。

**② `random.cpp` 曾被 sources 显式排除**,与那个宏配对。排除删掉。

**③ ⚠️ 默认后端在这里走不通,而它不会报错。** libc++ 对不认识的目标
落到 `_LIBCPP_USING_DEV_RANDOM`,那条要 `open("/dev/urandom")` ——
而能力型文件系统刻意不发绝对路径。必须在 `__config_site` 里**显式**选
`_LIBCPP_USING_GETENTROPY`:它不要描述符也不要路径。

**④ 声明找不到** —— musl 把 `getentropy` 放在 `<unistd.h>`,glibc 放在
`<sys/random.h>`,而 libc++ 只 include 后者。修在 openkal-musl 的 overlay
(见 mcpplibs/openkal-musl#6)。

⭐ 第三层最要紧:**那条默认路径在 openkal 上走不通,而它不报错,
只会在运行期打不开设备。**

实测,完整链路全程走接口层:

    std::random_device → getentropy → getrandom → kal_random_fill → 后端

    x86_64-linux    random_device: 3415089597 356442272   differ = true
    x86_64-windows  random_device: 1945321362 2880234856  differ = true (wine)

`entropy() = 0` 是 libc++ GETENTROPY 后端的既定行为,不是缺陷。

Author: Sunrisepeak

llvm-generated/generic/__config_site

@@ -47,7 +47,22 @@
 /* #undef _LIBCPP_NO_VCRUNTIME */
 /* #undef _LIBCPP_TYPEINFO_COMPARISON_IMPLEMENTATION */
 #define _LIBCPP_HAS_FILESYSTEM 1
-#define _LIBCPP_HAS_RANDOM_DEVICE 0
+#define _LIBCPP_HAS_RANDOM_DEVICE 1
+
+/* ⚠️ AND THE BACKEND, EXPLICITLY, BECAUSE THE DEFAULT ONE CANNOT WORK HERE.
+ *
+ * libc++ picks `_LIBCPP_USING_DEV_RANDOM` for anything it does not recognise,
+ * and that backend opens `/dev/urandom` by absolute path. A capability-oriented
+ * filesystem hands a program its roots rather than the whole namespace, so
+ * openkal.fs deliberately cannot open it --- the refusal is the model working.
+ *
+ * `getentropy` needs no descriptor and no path. Beneath it musl calls
+ * `getrandom`, which this port routes to `kal_random_fill` rather than to a
+ * system call, so the whole chain stays inside the interface layer:
+ *
+ *     std::random_device → getentropy → getrandom → openkal.random → backend
+ */
+#define _LIBCPP_USING_GETENTROPY 1
 #define _LIBCPP_HAS_LOCALIZATION 1
 #define _LIBCPP_HAS_UNICODE 1
 #define _LIBCPP_HAS_WIDE_CHARACTERS 1

mcpp.toml

@@ -253,12 +253,29 @@ sources = [
 
   # The standard library.
   "llvm/libcxx/src/*.cpp",
-  # openkal has no source of entropy and this port does not invent one, so the
-  # configuration sets _LIBCPP_HAS_RANDOM_DEVICE to 0 --- and the source that
-  # defines std::random_device must then not be compiled, which is what libc++'s
-  # own build does with the same switch. A program that names random_device is
-  # told by the linker, which is clause 6.1's report.
-  "!llvm/libcxx/src/random.cpp",
+  # ⭐ AND `random.cpp` IS BUILT, BECAUSE openkal NOW HAS A SOURCE OF ENTROPY.
+  #
+  # It used to be excluded, paired with `_LIBCPP_HAS_RANDOM_DEVICE 0`, and the
+  # note here said openkal had no source and this port would not invent one.
+  # That was accurate: a clock reading is unpredictable to a reader of the
+  # source and not to an adversary, and openkal.fs deliberately cannot open
+  # `/dev/urandom` --- a capability-oriented filesystem hands over roots rather
+  # than the whole namespace.
+  #
+  # ⚠️ Neither bypassing the interface layer nor inventing entropy was
+  # acceptable, so the layer gained an interface: `openkal.random`. The chain
+  # now runs entirely inside it:
+  #
+  #     std::random_device → getentropy → getrandom → kal_random_fill → backend
+  #
+  # ⚠️ THE BACKEND IS SELECTED EXPLICITLY IN `__config_site`. libc++ falls back
+  # to `_LIBCPP_USING_DEV_RANDOM` for anything it does not recognise, and that
+  # one opens `/dev/urandom` by absolute path. `_LIBCPP_USING_GETENTROPY` needs
+  # no descriptor and no path.
+  #
+  # ⚠️ The freestanding configuration keeps the switch at 0: a bare-metal
+  # backend provides `openkal.random` only if its board has a source, and
+  # clause 6.1 reports the absence at link time rather than here.
   "llvm/libcxx/src/filesystem/*.cpp",
   "llvm/libcxx/src/ryu/*.cpp",
   "llvm/libcxx/src/support/runtime/*.cpp",