vms-54e: LIB$GET_FOREIGN returns the foreign-command tail, not stdin

[baron-3dl]

Summary

Fixes LIB$GET_FOREIGN semantics (self-host MMK spine prereq, parent vms-801, per the vms-52c anchor docs/design-self-host-mmk-spine.md P5). It previously always read a line from stdin, so an image activated as a foreign command (FOO /qual target) could never retrieve its own invocation tail — the exact plumbing MMK needs (mmk.c calls lib$get_foreign to fetch its command line).

Semantics (grounded, clean-room)

Derived from the VSI OpenVMS RTL Library (LIB$) Manual and the corpus lib_get_foreign.c — no VSI/HPE source touched:

• First call returns the CLI foreign command line if present.
• No line (ordinary RUN, or an empty tail) and subsequent calls: prompt on SYS$INPUT exactly like LIB$GET_INPUT (RMS$_EOF at end of file).
• A line longer than the resultant descriptor is truncated → LIB$_INPSTRTRU.

Where the tail now comes from

DCL's foreign-command dispatch (dcl_exec_foreign_command) publishes the raw untokenized command tail (cmd->raw_tail) in the VMS_FOREIGN_CMD environment variable before activation — the same env channel DCL already uses for VMS process context (VMS_DEFAULT_DIR, ...). It reaches both activation models: in-process activation (same process) and the fork()+execve() fallback (inherited env). RUN and DCL-driven utilities do not set it, so they correctly fall through to SYS$INPUT.

lib$get_foreign consumes the line on first read by clearing the variable, modelling VMS's one-shot CLI foreign line in per-activation state — deliberately not a static flag, which would leak between images under in-process activation (the LIB$ code lives in the resident producer shareable whose statics persist across activations).

Changes

• libdef.h: add LIB$_INPSTRTRU (0x0015821C), grounded to the public $LIBDEF symbol table (FreeVMS starlet/src/libdef.h); encoding cross-checks against the already-verified LIB$_INSVIRMEM = 0x00158022.
• mk_decc_shr.sh: append unsetenv to the DECC$SHR symbol vector (append-only, GSMATCH-compatible; getenv/setenv were already exported). Caught by reproducing OVMX_LINK_NATIVE locally — the STRICT native link failed on the unresolved external until the vector was updated.
• test_lib_rtl_batch3.c: assert the exact tail is returned (not stdin), class-S space-padding, one-shot consumption (second call reads SYS$INPUT), and LIB$_INPSTRTRU truncation; make the EOF test deterministic by clearing VMS_FOREIGN_CMD first.

Test

• Debug ctest: green — 146 run/pass, 12 docker-e2e skipped (exit 0).
• x86_64 OVMX_LINK_NATIVE link_native_graph reproduced clean in alpine-musl: 9 artifacts, all EM_X86_64, zero DT_NEEDED.

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