feat(overlay): show places gained since the start in the OBS tower

Heats and finals started together show ▲/▼ against the start order
(new OBS option, on by default). In practice the best lap is no longer
printed twice as both result and best.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/run.js

@@ -2,6 +2,7 @@ import "./auth.test.js";
 import "./prologue_logic.test.js";
 import "./suspect_laps_logic.test.js";
 import "./series_logic.test.js";
+import "./overlay_position_change.test.js";
 import "./tts.test.js";
 import "./piper_speech.test.js";
 import "./driver_profile_logic.test.js";