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>

build.sh

@@ -44,10 +44,23 @@ cd ../../
 
 MARCH="${CRAFTER_BUILD_MARCH:-native}"
 MTUNE="${CRAFTER_BUILD_MTUNE:-native}"
+# The bootstrap targets the host toolchain's own triple: x86_64-pc-linux-gnu
+# on glibc distros, x86_64-alpine-linux-musl on Alpine. Override with
+# CRAFTER_BUILD_TARGET. The per-target define (triple with '-' -> '_') is the
+# same one the self-hosted build emits, so the host-gated code paths match.
+TARGET="${CRAFTER_BUILD_TARGET:-$(clang++ -print-target-triple)}"
+TARGET_DEFINE="CRAFTER_BUILD_CONFIGURATION_TARGET_$(printf '%s' "$TARGET" | tr - _)"
 
-common_options="--target=x86_64-pc-linux-gnu -stdlib=libc++ -I./build/glslang -std=c++26 -O3 -march=$MARCH -mtune=$MTUNE -fprebuilt-module-path=./build -D CRAFTER_BUILD_CONFIGURATION_TARGET_x86_64_pc_linux_gnu -D CRAFTER_BUILD_CONFIGURATION_TYPE_EXECUTABLE -c"
+# 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 add /usr/include itself: that breaks libc++'s #include_next).
+LLVM_INCDIR="$(llvm-config --includedir 2>/dev/null || true)"
+case "$LLVM_INCDIR" in ""|/usr/include) LLVM_INCFLAG="" ;; *) LLVM_INCFLAG="-I$LLVM_INCDIR" ;; esac
 
-clang++ --target=x86_64-pc-linux-gnu -std=c++26 -stdlib=libc++ -O3 -march=$MARCH -mtune=$MTUNE -Wno-reserved-identifier -Wno-reserved-module-identifier --precompile /usr/share/libc++/v1/std.cppm -o ./build/std.pcm
+common_options="--target=$TARGET -stdlib=libc++ -I./build/glslang $LLVM_INCFLAG -std=c++26 -O3 -march=$MARCH -mtune=$MTUNE -fprebuilt-module-path=./build -D $TARGET_DEFINE -D CRAFTER_BUILD_CONFIGURATION_TYPE_EXECUTABLE -c"
+
+clang++ --target=$TARGET -std=c++26 -stdlib=libc++ -O3 -march=$MARCH -mtune=$MTUNE -Wno-reserved-identifier -Wno-reserved-module-identifier --precompile /usr/share/libc++/v1/std.cppm -o ./build/std.pcm
 
 clang++ $common_options -fmodule-output interfaces/Crafter.Build-Shader.cppm -o ./build/Crafter.Build-Shader.o
 clang++ $common_options -fmodule-output interfaces/Crafter.Build-Platform.cppm -o ./build/Crafter.Build-Platform.o
@@ -100,7 +113,7 @@ ar rcs ./lib/libcrafter-build.a \
     ./build/Crafter.Build-Progress_impl.o \
     ./build/Crafter.Build-Asset_impl.o
 
-clang++ -std=c++26 -stdlib=libc++ -O3 -march=$MARCH -mtune=$MTUNE -fuse-ld=lld \
+clang++ --target=$TARGET -std=c++26 -stdlib=libc++ -O3 -march=$MARCH -mtune=$MTUNE -fuse-ld=lld \
     -Wl,--export-dynamic \
     -L./build \
     ./build/Crafter.Build-Shader.o \