Adds golden trace tests for DisplayController.run() (test/test_run_loop_golden.py on a fake clock with fake plugins, 15 scenarios, fixtures in test/fixtures/run_loop_golden/) and moves twelve blocks of run() into named helpers (_dispatch_first_frame, _resolve_durations, _resolve_active_mode, _needs_high_fps, _advance_after_screen and others) with the traces identical before and after. docs/RUN_LOOP_REDESIGN.md describes the target structure. Hardware-checked on hdpi: Vegas late-frame rate unchanged in an ABBA A/B.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
