Explains the traffic-control job in ci.yml (priority labels, preemption
vs. bounded hold-back, safety invariants, and the known FIFO-runner
limitation) for developers new to the repo. Notes that #342 will refactor
this logic into a tested Python module, at which point this doc gets
updated.
Closes#343
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
