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/CrossArchAarch64.cpp

@@ -0,0 +1,86 @@
+/*
+Crafter® Build
+Copyright (C) 2026 Catcrafts®
+Catcrafts.net
+
+LGPL-3.0-only.
+
+End-to-end cross-arch build through the V2 pipeline:
+the fixture's project.cpp targets aarch64-linux-gnu with cfg.sysroot pointing at
+the Arch Linux ARM rootfs at /opt/aarch64-rootfs. crafter-build cross-compiles
+the C++ source (with the libc++ std module from the sysroot), produces a real
+aarch64 ELF, and qemu-aarch64 (with QEMU_LD_PREFIX pointing at the sysroot so it
+can find ld-linux-aarch64.so.1) executes it.
+*/
+
+import std;
+#include "TestUtil.h"
+namespace fs = std::filesystem;
+using namespace TestUtil;
+
+namespace {
+    bool ToolPresent(std::string_view name) {
+        std::string cmd = std::format("which {} > /dev/null 2>&1", name);
+        return std::system(cmd.c_str()) == 0;
+    }
+}
+
+int main() {
+    try {
+        const fs::path sysroot = "/opt/aarch64-rootfs";
+        if (!fs::exists(sysroot / "usr/share/libc++/v1/std.cppm")) {
+            std::println("(skipped: aarch64 sysroot missing at {} — see README)", sysroot.string());
+            return 0;
+        }
+        if (!ToolPresent("qemu-aarch64")) {
+            std::println("(skipped: qemu-aarch64 not on PATH)");
+            return 0;
+        }
+
+        fs::path projectRoot = fs::current_path();
+        fs::path src = projectRoot / "tests" / "fixtures" / "cross-arch-aarch64";
+        fs::path crafterBuild = projectRoot / "bin" / "crafter-build";
+
+        fs::path work = CopyFixtureToTemp("CrossArchAarch64", src);
+
+        // Build through the V2 pipeline. The fixture's project.cpp pins
+        // cfg.target = "aarch64-linux-gnu" and cfg.sysroot = "/opt/aarch64-rootfs".
+        auto build = RunInDir(work, std::format("'{}'", crafterBuild.string()));
+        if (build.exitCode != 0) {
+            std::println(std::cerr, "build failed (rc={}):\n{}", build.exitCode, build.output);
+            return 1;
+        }
+
+        fs::path artifact = work / "bin" / "aarch-hello-aarch64-linux-gnu-armv8-a" / "aarch-hello";
+        if (!fs::exists(artifact)) {
+            std::println(std::cerr, "expected artifact missing at {}\nbuild log:\n{}",
+                artifact.string(), build.output);
+            return 1;
+        }
+
+        // Defend against silent host-arch fallback.
+        auto probe = RunInDir(work, std::format("file '{}'", artifact.string()));
+        if (probe.output.find("ARM aarch64") == std::string::npos) {
+            std::println(std::cerr, "artifact is not ARM aarch64 ELF:\n{}", probe.output);
+            return 1;
+        }
+
+        // Run via qemu-aarch64. QEMU_LD_PREFIX tells qemu where the target's
+        // dynamic linker (ld-linux-aarch64.so.1) and shared libs live.
+        auto run = RunInDir(work, std::format(
+            "QEMU_LD_PREFIX={} qemu-aarch64 '{}'",
+            sysroot.string(), artifact.string()));
+        if (run.exitCode != 0) {
+            std::println(std::cerr, "qemu-aarch64 run failed (rc={}):\n{}", run.exitCode, run.output);
+            return 1;
+        }
+        if (run.output != "hi from 64-bit aarch64\n") {
+            std::println(std::cerr, "output mismatch:\n  expected: \"hi from 64-bit aarch64\\n\"\n  got: {:?}", run.output);
+            return 1;
+        }
+        return 0;
+    } catch (const std::exception& e) {
+        std::println(std::cerr, "test exception: {}", e.what());
+        return 1;
+    }
+}