interleave

Iterate multiple projections round-robin, yielding (name, item).

Each turn advances one projection's cursor; when a cursor's buffer is empty, pulling from it drives the pump once, which fans out to every subscribed projection log. Projections whose items aren't consumed on this turn sit in their own buffers only until the next turn reaches them, bounding memory by the skew between projection rates rather than letting any single log grow to the full run length.

Projections are exhausted independently; a projection that finishes early drops out of the rotation while others continue. The overall iterator ends once all named projections are done.