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/fixtures/diamond/B/B.cppm

@@ -0,0 +1,6 @@
+export module B;
+import X;
+
+export int BValue() {
+    return XValue() * 2;
+}

tests/fixtures/diamond/C/C.cppm

@@ -0,0 +1,6 @@
+export module C;
+import X;
+
+export int CValue() {
+    return XValue() * 3;
+}

tests/fixtures/diamond/X/X.cppm

@@ -0,0 +1,5 @@
+export module X;
+
+export int XValue() {
+    return 7;
+}

tests/fixtures/diamond/main.cpp

@@ -0,0 +1,9 @@
+import std;
+import B;
+import C;
+
+int main() {
+    if (BValue() != 14) return 1;  // X(7) * 2
+    if (CValue() != 21) return 1;  // X(7) * 3
+    return 0;
+}