new tests

tests/HelloWorld/fixture/main.cpp

@@ -0,0 +1,2 @@
+import std;
+int main() { return 0; }

tests/HelloWorld/main.cpp

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+import std;
+import Crafter.Build;
+namespace fs = std::filesystem;
+using namespace Crafter;
+
+// Smoke test for the local Crafter.Build library: construct a Configuration
+// for the fixture/ subdir, call Build() in-process, and assert the binary
+// was produced. The build engine being exercised here is whatever this
+// test exe was statically linked against — which, via the parent
+// project.cpp's .Dependencies({ crafterBuildLib.get() }), is the local
+// checkout's library form. Edits to implementations/*.cpp show up here on
+// the next test run with no install step required.
+int main() {
+    Configuration cfg;
+    cfg.path = fs::current_path() / "tests" / "HelloWorld" / "fixture";
+    cfg.name = "hello";
+    cfg.outputName = "hello";
+    cfg.target = HostTarget();
+    cfg.type = ConfigurationType::Executable;
+
+    std::array<fs::path, 0> ifaces = {};
+    std::array<fs::path, 1> impls = { "main" };
+    cfg.GetInterfacesAndImplementations(ifaces, impls);
+
+    std::unordered_map<fs::path, std::shared_future<BuildResult>> depResults;
+    std::mutex depMutex;
+    BuildResult r = Build(cfg, depResults, depMutex);
+    if (!r.result.empty()) {
+        std::println(std::cerr, "build failed: {}", r.result);
+        return 1;
+    }
+
+    fs::path bin = cfg.BinDir() / "hello";
+    if (!fs::exists(bin)) {
+        std::println(std::cerr, "binary not produced at {}", bin.string());
+        return 1;
+    }
+    return 0;
+}