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/BuildError.cpp

@@ -0,0 +1,46 @@
+/*
+Crafter® Build
+Copyright (C) 2026 Catcrafts®
+Catcrafts.net
+
+LGPL-3.0-only.
+*/
+
+import std;
+#include "TestUtil.h"
+namespace fs = std::filesystem;
+using namespace TestUtil;
+
+int main() {
+    try {
+        fs::path projectRoot = fs::current_path();
+        fs::path src = projectRoot / "tests" / "fixtures" / "build-error";
+        fs::path crafterBuild = projectRoot / "bin" / "crafter-build";
+
+        fs::path work = CopyFixtureToTemp("BuildError", src);
+
+        auto build = RunInDir(work, std::format("'{}'", crafterBuild.string()));
+        if (build.exitCode == 0) {
+            std::println(std::cerr, "expected nonzero exit, got 0; build output:\n{}", build.output);
+            return 1;
+        }
+
+        // diagnostic must surface the unresolved name; fragile-ish but recognizable
+        if (build.output.find("undefined_symbol_xyzzy_oqv") == std::string::npos) {
+            std::println(std::cerr, "diagnostic missing unresolved-name reference:\n{}", build.output);
+            return 1;
+        }
+
+        // and the artifact must NOT have been produced
+        fs::path artifact = work / "bin" / "broken-x86_64-pc-linux-gnu-native" / "broken";
+        if (fs::exists(artifact)) {
+            std::println(std::cerr, "artifact unexpectedly produced at {}", artifact.string());
+            return 1;
+        }
+
+        return 0;
+    } catch (const std::exception& e) {
+        std::println(std::cerr, "test exception: {}", e.what());
+        return 1;
+    }
+}