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>

README.md

@@ -18,6 +18,8 @@ Bootstrap requires `clang`, `cmake`, `git`, `lld`, and `libc++`.
 
 For distro-packaged installs, `crafter-build` finds its modules at `<prefix>/share/crafter-build/` automatically — no env var required.
 
+The bootstrap targets the host toolchain's own triple (`clang++ -print-target-triple`): `x86_64-pc-linux-gnu` on glibc distros, `x86_64-alpine-linux-musl` on Alpine/postmarketOS (`apk add clang clang-dev lld libc++-dev llvm-libunwind-dev llvm-runtimes llvm llvm-dev cmake build-base python3 git`). Override with `CRAFTER_BUILD_TARGET=`. CI ships both: `crafter-build-linux-x86_64-v{2,3,4}.tar.gz` (glibc) and `crafter-build-linux-x86_64-musl-v2.tar.gz` (musl, needs `libc++` at runtime — the launcher dlopens `project.so`, so it cannot be static).
+
 To build the system as a distro package on Arch:
 
 ```bash