Add spoken narration to the video guide recorder

The clips were silent screen captures - the user's actual ask was for someone talking
through what's happening. Each clip step now carries a narration line synthesized
offline via espeak-ng; the recording paces itself to each line's real spoken duration,
and the final audio track is built from the real measured per-line start times (adelay +
amix) so narration and on-screen action stay in sync over a 30+ second clip instead of
drifting against an idealized schedule.

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

package.json

@@ -1,6 +1,6 @@
 {
   "name": "rc-racecore",
-  "version": "1.18.0",
+  "version": "1.18.1",
   "private": true,
   "description": "RC RaceCore live timing and race control with AMMC ingest and SQLite persistence",
   "engines": {