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/QemuUser.cpp

@@ -0,0 +1,65 @@
+/*
+Crafter® Build
+Copyright (C) 2026 Catcrafts®
+Catcrafts.net
+
+LGPL-3.0-only.
+*/
+
+import std;
+#include "TestUtil.h"
+namespace fs = std::filesystem;
+using namespace TestUtil;
+
+namespace {
+    bool Contains(std::string_view haystack, std::string_view needle) {
+        return haystack.find(needle) != std::string_view::npos;
+    }
+
+    std::string PickQemu() {
+        if (const char* v = std::getenv("CRAFTER_TEST_QEMU"); v && *v) return v;
+        return "qemu-x86_64";
+    }
+
+    bool QemuPresent(const std::string& qemu) {
+        std::string cmd = std::format("which {} > /dev/null 2>&1", qemu);
+        return std::system(cmd.c_str()) == 0;
+    }
+}
+
+int main() {
+    try {
+        std::string qemu = PickQemu();
+        if (!QemuPresent(qemu)) {
+            std::println("(skipped: {} not on PATH)", qemu);
+            return 0;
+        }
+
+        fs::path projectRoot = fs::current_path();
+        fs::path src = projectRoot / "tests" / "fixtures" / "qemu-runner";
+        fs::path crafterBuild = projectRoot / "bin" / "crafter-build";
+
+        fs::path work = CopyFixtureToTemp("QemuUser", src);
+
+        // Tell the inner crafter-build to use qemu for the host triple via the
+        // FromEnv mechanism that the fixture's project.cpp opted into.
+        auto run = RunInDir(work, std::format(
+            "CRAFTER_BUILD_RUNNER_x86_64_pc_linux_gnu='qemu:{}' '{}' test",
+            qemu, crafterBuild.string()));
+
+        if (run.exitCode != 0) {
+            std::println(std::cerr, "inner runner failed (rc={}):\n{}", run.exitCode, run.output);
+            return 1;
+        }
+        if (!Contains(run.output, std::format("\xE2\x9C\x85 Hello (qemu:{})", qemu))) {
+            std::println(std::cerr,
+                "expected '✅ Hello (qemu:{})' marker not found in inner output:\n{}",
+                qemu, run.output);
+            return 1;
+        }
+        return 0;
+    } catch (const std::exception& e) {
+        std::println(std::cerr, "test exception: {}", e.what());
+        return 1;
+    }
+}