Turns [{name, ops}] -- one label-relative op list per token or rule, as produced by Grammar.VM.Compiler -- into a single linked Grammar.VM.Program.

Each op list may contain two kinds of placeholder alongside real instructions: {:label, n} (a jump target local to that op list) and the real instructions {:jmp, n} / {:choice, n} / {:commit, n} / {:back_commit, n} / {:test_progress, n} that reference one. {:call, name} is a different kind of forward reference -- to another token/rule's own entry point, by name, resolved against every op list at once (that's the whole reason linking happens globally instead of per-name: :call needs to reach targets compiled by a different, unrelated compile/1 call). {:custom, module, function, dep_names} (a Grammar.IR.Custom @native(...) node) resolves the same way, except it names several targets at once -- one per declared dependency -- ending up as {:custom, module, function, %{dep_name => entry_index}}.