Koru Standard Library: the `explain` part of pump — joined via `~part explain` in pump.kz; members land in koru_pump. The pump is the most distributed surface in the stdlib: participants join from anywhere via `std/pump(name) ! step |> ...` and `run` enumerates them by generated-decl name. Nothing in any single file answers "who joined, which verbs each answers, does this pump ever drain" — the explainer folds the source sites in document order with the SAME checks the create/join/run transforms apply; a program that refuses still explains itself, each refusal verbatim via the shared `PM` messages. Callee spellings come from PH.lastInvocation — the same read the join transform uses — and `PS.synthesized` names callees the store transform will generate.

// @retain: explain_pump_event is referenced from generated code
// (koruc's koru_explain_gather invokes every discovered [explainer]).
~[comptime|explainer] pub tor explain-pump {
    program: std/compiler:*const Program,
    allocator: std.mem.Allocator
} -> ExplainReport