fix(test): 护栏矩阵不能把「本机是 Linux」当成前提;doctor 呈现降级 note

CI 的 macOS 与 Windows runner 上 RuntimeSearchClosure 两条红:它们用空 triple 断言
「拿得到 farm」,而空 triple 的意思是**本宿主的格式** —— 在 macOS/Windows 上那正是
「没有 ELF 搜索路径」,也就是这条护栏本身要表达的东西,不是它的例外。

改为:显式 Linux target 在任何 runner 上都断言拿得到(护栏读的是目标,不是 runner);
空 triple 那条按本机格式给期望值,并把理由写进断言消息。

顺带补上设计里承诺而没实现的一处:`mcpp why runtime` 现在呈现 binding 的
`declared` 与 `note`。只在发生那次构建里出现过一次的降级,是事后查不到的降级 ——
而「我的 verdict 为什么是 inconclusive」正是这个命令存在的理由。

Author: speak-agent

src/doctor.cppm

@@ -592,10 +592,19 @@ int print_stored_runtime_resolution() {
     std::println("runtime resolution: {}", path.string());
     if (auto binding = runtime->find("binding");
         binding != runtime->end() && binding->is_object()) {
-        std::println("binding: {} via {} (contract {})",
-            binding->value("runtime_id", "?"),
+        const bool declared = binding->value("declared", true);
+        std::println("binding: {} via {} (contract {}){}",
+            declared ? binding->value("runtime_id", "?") : "(undeclared)",
             binding->value("provider_id", "?"),
-            binding->value("contract_hash", "?"));
+            binding->value("contract_hash", "?"),
+            declared ? "" : "  — this SubOS did not describe itself");
+        // The note, when there is one. A degradation that only ever appeared
+        // once during the build it happened in is a degradation nobody can look
+        // up afterwards, and "why is my verdict inconclusive" is exactly the
+        // question this command exists to answer.
+        if (auto note = binding->value("note", std::string{}); !note.empty())
+            for (auto line : std::views::split(note, '\n'))
+                std::println("  {}", std::string_view(line));
     }
 
     std::println("requirements:");

tests/unit/test_runtime_contract.cpp

@@ -325,10 +325,18 @@ std::size_t farm_entries(const std::vector<mcpp::platform::search::Dir>& dirs) {
 }  // namespace
 
 TEST(RuntimeSearchClosure, HostTargetGetsTheFarm) {
-    EXPECT_EQ(farm_entries(build::runtime_search_closure(
-        plan_for(""), host_binding_with_farm())), 1u);
+    // An EXPLICIT Linux target is a farm target from anywhere, including a
+    // macOS or Windows runner: the guards read the target, not the runner.
     EXPECT_EQ(farm_entries(build::runtime_search_closure(
         plan_for("x86_64-linux-gnu"), host_binding_with_farm())), 1u);
+
+    // An EMPTY triple means "this host", so the answer is the host's format —
+    // and that is the point of the ELF guard, not an exception to it. Asserting
+    // 1 unconditionally is what failed on the macOS and Windows runners.
+    const std::size_t expectedForHost = mcpp::platform::is_linux ? 1u : 0u;
+    EXPECT_EQ(farm_entries(build::runtime_search_closure(
+        plan_for(""), host_binding_with_farm())), expectedForHost)
+        << "an empty triple must resolve to this host's format";
 }
 
 TEST(RuntimeSearchClosure, CrossTargetGetsNoFarm) {
@@ -355,7 +363,8 @@ TEST(RuntimeSearchClosure, UndeclaredBindingStillGetsItsOwnFarm) {
     auto b = host_binding_with_farm();
     b.declared = false;
     b.runtimeId.clear();
-    EXPECT_EQ(farm_entries(build::runtime_search_closure(plan_for(""), b)), 1u);
+    EXPECT_EQ(farm_entries(build::runtime_search_closure(
+        plan_for("x86_64-linux-gnu"), b)), 1u);
     // …but the architecture guard still applies: unknown libc is not a licence
     // to ignore everything else.
     EXPECT_EQ(farm_entries(build::runtime_search_closure(