Adds a fast traffic-control-tests job (ubuntu, actions/checkout +
actions/setup-python, no emulator/Gradle) that runs the 37 pure-stdlib
unit tests for .github/scripts/traffic_control.py on every PR, and
wires it into ci-passed's needs so a regression blocks merge instead
of only being caught locally.
Closes#346
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>