030 spawned producer send

OWED-PROBE: can a `worker.spawn`'d thread invoke a channel's generated send path? The emitted ring (`__koru_chan_<n>_<kind>_ring`, MpmcRing global in the emitted module) is CAS-safe by construction; what's unmeasured is module reachability — whether a lib `~proc|zig` body (or a spawned work_fn) can see the generated unit/global, and whether `send`'s verdicts dispatch sanely off-thread. This is the eventfd bridge's precondition: cross-thread producer → ring → pump `! wait` eventfd wake. PIN when an input spelling exists — candidate shapes: spawned `~proc` calling the generated `__channel_send_<n>_<kind>` unit, or a channel-level `produce` verb that spawns internally.