vms-947: implement sys$setddir (self-host MMK RTL prereq)

[baron-3dl]

Summary

Implements SYS$SETDDIR (previously absent) — reads and optionally changes the process default directory string, the [dir] used to resolve relative file specifications. A hard RTL prereq for native MMK.EXE (parent vms-801; design anchor vms-52c / docs/design-self-host-mmk-spine.md). The vendored MadGoat MMK (tests/corpus/tier3-mmk/mmk.c) is a real consumer.

Semantics (clean-room, Rule 8)

Derived only from the OpenVMS System Services Reference ($SETDDIR) and the calling convention MMK exercises:

• sys$setddir(new_dir, old_len, old_dir)
  • new_dir NULL → read-only (return current directory)
  • old_dir supplied → receive previous string (truncated to buffer); old_len = bytes written (MMK does old_def_dir[len]='\0')
  • old_len without old_dir → full current length
  • previous captured before the change (VMS order)
  • empty / over-long new spec → SS$_BADPARAM (no fake success — INV-DCL/INV-6)

Coordination with existing default-dir storage

OVMX keeps the process default directory in the PCB (pcb->default_dir) — the same store src/libvms/syssvc/sys_assign.c reads to resolve SYS$DISK / relative specs. sys$setddir reads and mutates that store via vms_pcb_get / vms_pcb_set_default_dir, so a change is observable by subsequent relative filespec resolution — not a per-call no-op. OVMX conflates device+directory into the one field; a bare bracketed [dir] keeps the current device prefix (mirrors SET DEFAULT / cmd_set_default), a full device:[dir] spec is stored verbatim. Declared OVMX-USERSPACE in the sys_misc.c register banner (the default directory is a per-process construct on real VMS too; no executive resource is faked) — test_userspace_service_register.sh passes.

Native-link

sys$setddir is a global text symbol in sys_misc (already in mk_libvms_shr.sh LIST), so it auto-exports into LIBVMS\$SHR.EXE's .vms\$sv vector. Verified via the alpine-musl OVMX_LINK_NATIVE reproduction: 9 artifacts, EM_X86_64, zero DT_NEEDED, and sys$setddir present in the .vms\$sv section.

Test

tests/libvms/test_setddir.c (test_libvms_setddir) — 19 assertions: read-only, set + return-previous, round-trip observability, full vs bracketed spec handling, set-only form, and both SS$_BADPARAM paths.

Verification

• Debug ctest: exit 0 — 147 tests passed (12 e2e/QEMU-install tests skipped as standard), including test_libvms_setddir.
• Native-link (x86_64 alpine-musl link_native_graph): green, symbol in .vms\$sv.
• Userspace service register gate: PASS.

No starlet.h prototype added (matches existing convention — sys$setprv/sys$getsyi have none; MMK declares its own; native-link binds by symbol) to avoid perturbing the corpus conformance baseline.

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