musl host support: bootstrap and run on Alpine (x86_64-alpine-linux-musl)

The Linux host code paths were gated on the x86_64_pc_linux_gnu target
define alone, and LoadProject hardcoded --target=x86_64-pc-linux-gnu when
compiling project.cpp and the module PCMs. Both break on musl hosts: the
glibc release launcher cannot run there at all (no glibc loader; gcompat
lacks libgcc_s and the __isoc23_* symbols), and a musl-built launcher
would still compile project.cpp for the wrong libc.

- The host gates test clang's __linux__ instead of one triple's define:
  they mean "Linux host", whatever the libc (or, later, the CPU).
- LoadProject derives the host triple from HostTarget() (clang's
  -print-target-triple), which is also more correct on glibc distros
  whose triple isn't x86_64-pc-linux-gnu.
- project.cpp / the cmake libc++ flags treat any *-linux-gnu or
  *-linux-musl target as Linux.
- build.sh targets the host toolchain's triple (CRAFTER_BUILD_TARGET
  overrides) and derives the per-target define from it. On Arch this is
  byte-for-byte the previous behaviour.
- CI: a release-musl job bootstraps on alpine:edge, runs the tests, and
  attaches crafter-build-linux-x86_64-musl-v2.tar.gz to the rolling
  latest release (via the API, so the glibc assets survive). Dynamic
  against musl libc++: the launcher dlopens project.so, so it cannot be
  fully static.

First consumer: imsd's package CI, which cross-compiles for aarch64 from
an Alpine container.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

project.cpp

@@ -32,6 +32,18 @@ extern "C" Configuration CrafterBuildProject(std::span<const std::string_view> a
 
     CrafterBuildLib->dependencies = { math, asset };
     CrafterBuildLib->defines.push_back({"CRAFTER_BUILD_HAS_ASSET", ""});
+    // Crafter.Build-Lint includes libclang's clang-c/Index.h. glibc distros
+    // keep it in /usr/include; Alpine keeps LLVM under /usr/lib/llvmNN, so ask
+    // llvm-config and add the dir when it is not already on the default path
+    // (never /usr/include itself: that breaks libc++'s #include_next).
+    if (CrafterBuildLib->target.ends_with("-linux-gnu") || CrafterBuildLib->target.ends_with("-linux-musl")) {
+        auto llvmInc = RunCommandWithTimeout("llvm-config --includedir 2>/dev/null", std::chrono::seconds(10));
+        std::string incDir = llvmInc.output;
+        while (!incDir.empty() && (incDir.back() == '\n' || incDir.back() == '\r')) incDir.pop_back();
+        if (llvmInc.exitCode == 0 && !incDir.empty() && incDir != "/usr/include") {
+            CrafterBuildLib->compileFlags.push_back(std::format("-I{}", incDir));
+        }
+    }
     {
         std::array<fs::path, 11> interfaces = {
             "interfaces/Crafter.Build",
@@ -87,7 +99,9 @@ extern "C" Configuration CrafterBuildProject(std::span<const std::string_view> a
         std::array<fs::path, 1> implementations = { "implementations/main" };
         cfg.GetInterfacesAndImplementations(interfaces, implementations);
     }
-    if (cfg.target == "x86_64-pc-linux-gnu") {
+    // Linux hosts, glibc or musl (Alpine): -ldl is a libc stub on musl, harmless.
+    const bool linuxTarget = cfg.target.ends_with("-linux-gnu") || cfg.target.ends_with("-linux-musl");
+    if (linuxTarget) {
         cfg.linkFlags.push_back("-Wl,--export-dynamic");
         cfg.linkFlags.push_back("-ldl");
     }
@@ -102,7 +116,7 @@ extern "C" Configuration CrafterBuildProject(std::span<const std::string_view> a
     // links each test exe against CrafterBuildLib built from the local
     // sources — so the code under test is whatever's in this checkout.
     // Mirrors how downstream consumers link their own libraries into tests.
-    if (cfg.target == "x86_64-pc-linux-gnu") {
+    if (linuxTarget) {
         cfg.AddTest("HelloWorld").Dependencies({ CrafterBuildLib.get() });
         cfg.AddTest("StaticLib").Dependencies({ CrafterBuildLib.get() });
         cfg.AddTest("ModuleInterface").Dependencies({ CrafterBuildLib.get() });