plg-runtime: the support library linked into every compiled Prolog
binary (libplg_runtime.a).
It provides the machine substrate compiled predicates call into —
term heap, trail, choice-point stack, generic unification, the
goal-only --query parser, solution output, and the process entry
point. It contains NO clause interpreter: clause control flow lives
in the LLVM IR that plgc generates (see
docs/design/COMPILATION_MODEL.md and docs/design/LESSONS_FROM_V1.md).