fix(freestanding): 直连链接下的共享库,用话说清楚而不是丢一条 ninja 规则

直连链接路径定义 cxx_link / c_link / cxx_archive,不定义共享库规则 —— 没有加载器
的目标没有东西去加载它。缺这条检查时,失败长这样:

    ninja: error: build.ninja:88: unknown build rule 'cxx_shared'

对一个在清单里写下 kind = "shared" 的人,这句话指的是一条内部规则。

⚠️ 补上那条规则比省掉它更糟:ld.lld -shared 会成功,产出一个那台机器上没有任何
东西能加载的对象 —— 静默的无意义,而不是响亮的拒绝。

e2e 137 加了第 7 条断言,同时检查两件事:拒绝时提到了原因,且**没有**提到内部
规则名。

Author: speak-agent

.agents/docs/2026-08-21-freestanding-outstanding-four.md

@@ -272,10 +272,22 @@ libcxx-headers = "22.1.8"     # 与产出导出表的那个 libc++ 同版本
 
 **步骤 2:新建 `xim:libcxx-headers`。**
 
-内容是 `include/c++/v1` 与 `share/libc++`,实测 xz 压缩后 **0.9 MB**。它与
-宿主无关,因此**一个包服务全部五个宿主**,而「加进载荷」要给每个缺失的宿主
+内容是 `include/c++/v1` 与 `share/libc++`。复测于 llvm 22.1.8 的载荷:
+
+| | |
+|---|---|
+| `include/c++/v1` | 16 MB |
+| `share/libc++` | 624 KB |
+| 打包后(`tar` + `xz -9`) | **943,180 字节** |
+
+它与宿主无关,因此**一个包服务全部五个宿主**,而「加进载荷」要给每个缺失的宿主
 各加一次。
 
+⚠️ **这个包尚未上传,而不上传是有意的。** 它今天可以打出来,但没有任何东西验证过
+它在 Windows 上确实让 `std-freestanding` 工作 —— 步骤 1 还没做。把一个未经验证的
+产物先镜像出去,只是让「未经验证的东西」传播得更快。这与 `mcpplibs/qemu-x86` 那边
+定下的顺序是同一条:**先跑通,再发布**。
+
 **步骤 3:实现层的选择成为可声明的。**
 
 接口层不变,配置层按检测到的实现分支:

src/build/ninja_backend.cppm

@@ -2306,6 +2306,33 @@ std::expected<BuildResult, BuildError> NinjaBackend::build(const BuildPlan& plan
             "compile_commands.json was not updated: {}", cdb.error().message));
     }
 
+    // ⚠️ A SHARED LIBRARY ON A TARGET WHOSE LINK IS DRIVEN BY THE LINKER, SAID
+    // IN WORDS RATHER THAN AS A MISSING NINJA RULE.
+    //
+    // The direct-linker path defines `cxx_link`, `c_link` and `cxx_archive` and
+    // no shared-library rule, because a target with no loader has nothing to
+    // load one. Without this check the failure is
+    //
+    //     ninja: error: build.ninja:88: unknown build rule 'cxx_shared'
+    //
+    // which names an internal rule to somebody who wrote `kind = "shared"` in a
+    // manifest. Defining the rule would be worse: `ld.lld -shared` succeeds and
+    // produces an object nothing on that machine can load.
+    if (!flags.ldDriver.empty()) {
+        for (auto const& lu : plan.linkUnits) {
+            if (lu.kind != LinkUnit::SharedLibrary) continue;
+            return std::unexpected(BuildError{std::format(
+                "target '{}' is a shared library, and '{}' has no dynamic "
+                "loader to load one.\n"
+                "  A freestanding image is linked statically because there is "
+                "no other option: nothing on that machine resolves a symbol at "
+                "run time.\n"
+                "  Use `kind = \"lib\"` for a static library, or `kind = "
+                "\"bin\"` for the image itself.",
+                lu.targetName, plan.toolchain.targetTriple)});
+        }
+    }
+
     // A distribution contract that could not be honored is reported, never
     // silently downgraded — the whole point of the model (INV-1/INV-4 in
     // .agents/docs/2026-08-02-issue336-pr142-analysis.md). Emitted here rather

tests/e2e/137_x86_64_zero_libc_target.sh

@@ -141,4 +141,39 @@ grep -q -- "-march=x86-64" verbose.log || {
 grep -q -- "-mno-red-zone" verbose.log || {
     echo "the red zone was left enabled on a target that takes interrupts"; exit 1; }
 
+# ── 7. A shared library on this target is refused in words ─────────────────
+#
+# ⚠️ THE DIRECT-LINKER PATH DEFINES NO SHARED-LIBRARY RULE, AND WITHOUT A CHECK
+# THE FAILURE NAMES AN INTERNAL ONE:
+#
+#     ninja: error: build.ninja:88: unknown build rule 'cxx_shared'
+#
+# to somebody who wrote `kind = "shared"` in a manifest. Defining the rule would
+# be worse than omitting it: `ld.lld -shared` succeeds and produces an object
+# nothing on a machine with no loader can load.
+cd "$TMP"
+mkdir -p shared/src && cd shared
+cat > mcpp.toml <<'EOF'
+[package]
+name    = "shr"
+version = "0.1.0"
+
+[build]
+target = "x86_64-none-elf"
+
+[targets.shr]
+kind = "shared"
+EOF
+echo 'int f() { return 1; }' > src/lib.cpp
+if "$MCPP" build > shared.log 2>&1; then
+    cat shared.log; echo "a shared library was built for a target with no loader"; exit 1
+fi
+grep -q "no dynamic loader" shared.log || {
+    cat shared.log
+    echo "the refusal does not name the situation"; exit 1; }
+if grep -q "unknown build rule" shared.log; then
+    cat shared.log
+    echo "the failure names an internal ninja rule instead of the cause"; exit 1
+fi
+
 echo "PASS: x86_64-none-elf builds on the zero-libc tier, linked by ld.lld directly"