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>

implementations/Crafter.Build-External.cpp

@@ -137,8 +137,8 @@ std::string BuildInjectedCMakeFlags(const fs::path& cmakeBuildDir, std::string_v
     // itself is built with -stdlib=libc++); mingw cross-compile uses libstdc++
     // shipped by the mingw-w64 toolchain so we leave the default; Windows
     // native (msvc target) similarly uses libc++ via LIBCXX_DIR.
-    #ifdef CRAFTER_BUILD_CONFIGURATION_TARGET_x86_64_pc_linux_gnu
-    if (target == "x86_64-pc-linux-gnu") {
+    #ifdef __linux__
+    if (target.ends_with("-linux-gnu") || target.ends_with("-linux-musl")) {
         out += " -DCMAKE_CXX_FLAGS=-stdlib=libc++";
         out += " -DCMAKE_EXE_LINKER_FLAGS=-stdlib=libc++";
         out += " -DCMAKE_SHARED_LINKER_FLAGS=-stdlib=libc++";