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>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "rc-racecore",
-  "version": "1.38.0",
+  "version": "1.39.0",
   "private": true,
   "description": "RC RaceCore live timing and race control with AMMC ingest and SQLite persistence",
   "engines": {