core: build and run on musl targets

[congwang-mk]

cargo build --target x86_64-unknown-linux-musl failed with 19 type errors, so anyone wanting a static binary for a small Alpine image had to fall back to a glibc base. The errors come from the libc crate typing the bindings differently per target, not from anything a build flag can change. Fixing the build then exposed two runtime bugs that only show on musl.

core: build on musl targets. The libc crate types the ptrace request as c_uint on glibc and c_int on musl, ioctl requests as c_ulong versus c_int, and msg_controllen as size_t versus u32. The explicit glibc-shaped casts are dropped so the constants carry their per-target type, ioctl requests cast to libc::Ioctl, and the one helper that names the ptrace request type sits behind a cfg alias. A test spelled STATX_BASIC_STATS locally because the libc crate only exports it for glibc.

landlock: open rule paths with open(2), not OpenOptions. musl defines O_SEARCH as O_PATH and folds it into O_ACCMODE, and std's OpenOptions masks custom flags with !O_ACCMODE. On musl the Landlock rule open silently lost O_PATH and became a real O_RDONLY open, so a read rule on a FIFO blocked child setup forever. Calling open(2) directly keeps O_PATH on every libc.

seccomp: stop the exec rewrite scan at the first NUL. The scan for argv/envp strings that could run into the execve path buffer read the whole span up to the buffer before looking for a NUL. glibc's brk heap keeps that span mapped, but musl's allocator places allocations in separate mmap chunks with unmapped pages between them, so the read hit a hole and the child's execve failed with EFAULT under chroot (19 of 20 runs). Checking each page for a NUL before reading the next also matches what the kernel would copy.

Verified on x86_64: the musl CLI is a static-pie binary that passes sandlock check and enforces Landlock; the full workspace suite on musl and on gnu produce identical results, with the only failures being the environment-dependent tests addressed in #223 plus one hardlink test that fails the same way on main here.

Not covered: the FFI cdylib is not produced for musl unless built with -C target-feature=-crt-static, and no musl target is added to the release matrix yet.

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