vms-706: build vms_stdio.c + vms_futex.c on VAX (close the freestanding-facility gap)

[baron-3dl]

The gap (CONVERGE #3)

The freestanding buffered-I/O (vms_stdio.c) and futex (vms_futex.c) facilities built on x86_64/aarch64/alpha but were absent on VAX: the NetBSD syscall header omitted vms_sys_openat/vms_sys_futex, vms_types.h left VMS_FUTEX_* undefined on NetBSD, and both files were excluded from the netbsd build set — any feature resting on them was silently VAX-absent. Converged by providing the wrappers (max-shared-code directive), not fencing the feature off VAX.

Changes (3 files)

• arch/vax/vms_syscall_netbsd.h — add vms_sys_openat (NetBSD libc openat(2); flags is the VMS_O_* mask) and vms_sys_futex. NetBSD ships no libc futex() — the syscall is __futex (SYS___futex), a 7-arg form that inserts val2 before val3 (unused by our WAIT/WAKE/WAIT_BITSET ops → 0); called via syscall(SYS___futex, …). The wrapper folds errno into a raw negative-errno return so vms_futex.c's vms_condvar_timedwait — which tests ret == -VMS_ETIMEDOUT — keeps working (the libc -1/errno convention would break timeout detection).
• vms_types.h — add a __NetBSD__ branch defining VMS_FUTEX_* as aliases of <sys/futex.h> FUTEX_* (substrate-correct, _Static_assert-checked — never a transcribed Linux number, exactly what the prior "Linux-only" note guarded against). VMS_O_* was already NetBSD-mapped.
• CMakeLists.txt — add both files to the netbsd VMSSYS_C_SOURCES.

Safety + proof

• The Linux/host path is untouched (both additions are __NetBSD__-guarded) — host tests unaffected.
• Both TUs cross-compile clean to elf32-vax against the pinned NetBSD 10.1 sysroot (-Wall -Wextra, static-asserts pass, U openat / U syscall referenced). The vax-cmake-images CI gate now compiles them.
• Runtime SIMH verification of the futex wrapper deferred (host RAM-blocked); the __futex 7-arg ABI is verified against NetBSD syscalls.master.

Shared-core convergence — rides the 3-way width gate.

🤖 Generated with Claude Code