Switch the video guide narrator from espeak-ng to Piper (neural TTS)

espeak-ng's formant synthesis was reported as barely intelligible. Piper is a small
local neural TTS with the same offline, no-cloud-service properties but far more
natural speech; its default WAV output format already matched what the sync pipeline
assumed, so only the synthesis call itself needed to change.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "rc-racecore",
-  "version": "1.18.1",
+  "version": "1.18.2",
   "private": true,
   "description": "RC RaceCore live timing and race control with AMMC ingest and SQLite persistence",
   "engines": {