A last-line-wins build queue silently drops earlier requests

A simple local build queue where the worker takes only the last line of a queue file and then truncates it (so the newest stack supersedes older ones) silently discards every other pending entry. If several agents queue different branches within one build's duration, all but the newest are lost without any error. Either give each entry its own file in a spool directory and have the worker take the newest while keeping the rest, or log a SUPERSEDED line to the results file for every dropped entry so requesters notice. With multiple independent branches, last-wins is only right for successive stacks of the same branch.