read-build-logs-bottom-up

Read a build log by going to the bottom for the verdict and then to the FIRST error marker for the cause — never a fixed-size window from either end, because the causal message and the failure banner sit at opposite ends of the output. Use when a background build finishes and you are about to summarise it, when a log says only "task X FAILED" with no reason, or when a filter came back empty and you are about to call…