Spawn/wait fixture #

This deterministic AArch64 Linux fixture uses the runtime's constrained clone(CLONE_VM | CLONE_VFORK | SIGCHLD) mode. The child verifies PID 2 and parent PID 1, writes child, and exits 7. The restored parent verifies PID 1, reaps PID 2 with wait4, checks the encoded exit status, writes parent, and exits zero.

Rebuild it from the repository root while keeping the Zig cache local:

TMPDIR="$PWD/.tmp" guest-tests/spawn-wait/build.sh