test runner, cross-target runners, lib/exe split

- subprocess-isolated test runner (replaces V1 dlopen-RunTest);
  Pass/Fail/Crash/Timeout/Skipped outcomes via :Test partition
- TestRunner abstraction with command templates: Local, Ssh,
  SshWin (cmd.exe-shell), QemuUser, FromEnv; probe-based skip
  when runner unreachable
- transitive PCM-path propagation in Build(); resolveImport
  walks deps recursively; depResults cache keyed by PcmDir()
  so per-target builds don't collide
- cfg.sysroot threaded through BuildStdPcm + base compile/link
  command (enables aarch64 cross via Arch Linux ARM rootfs)
- lib + exe split: project.cpp defines crafterBuildLib
  (LibraryStatic) + crafterBuildExe (Executable depending on
  it); build.sh produces lib/libcrafter-build.a alongside
  bin/crafter-build for downstream static-link consumers
- Windows DLL+launcher: CRAFTER_API macro, /EXPORT flag for
  project.dll's CrafterBuildProject; Crafter::Run as the real
  entry point with main.cpp as a thin wrapper
- 18 tests: HelloWorld/WithModule/Defines/CrossProjectModule/
  Diamond × (Linux + sshwin:winvm), plus Incremental,
  BuildError, Libraries, RunnerClassification, QemuUser,
  SshRunner, WindowsViaSsh, CrossArchAarch64
- single ./bin/crafter-build test runs everything; Windows
  variants skip gracefully if winvm SSH alias unreachable

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/TestUtil.h

@@ -0,0 +1,36 @@
+#pragma once
+
+namespace TestUtil {
+    inline std::string ReadFile(const std::filesystem::path& p) {
+        std::ifstream f(p);
+        std::stringstream ss;
+        ss << f.rdbuf();
+        return ss.str();
+    }
+
+    inline std::filesystem::path CopyFixtureToTemp(std::string_view testName, const std::filesystem::path& source) {
+        namespace fs = std::filesystem;
+        fs::path tmp = fs::temp_directory_path() / std::format("crafter-test-{}", testName);
+        fs::remove_all(tmp);
+        fs::copy(source, tmp, fs::copy_options::recursive);
+        return tmp;
+    }
+
+    struct CmdResult {
+        int exitCode;
+        std::string output;
+    };
+
+    inline CmdResult RunInDir(const std::filesystem::path& cwd, std::string_view command) {
+        namespace fs = std::filesystem;
+        // Log inside cwd so parallel test drivers don't trample each other.
+        fs::path log = cwd / ".crafter-cmd-output.log";
+        std::string cmd = std::format("cd '{}' && {} > '{}' 2>&1",
+            cwd.string(), command, log.string());
+        int rc = std::system(cmd.c_str());
+        std::string out = ReadFile(log);
+        std::error_code ec;
+        fs::remove(log, ec);
+        return {rc, std::move(out)};
+    }
+}