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-lock.json

@@ -1,12 +1,12 @@
 {
   "name": "rc-racecore",
-  "version": "1.18.0",
+  "version": "1.18.1",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "rc-racecore",
-      "version": "1.18.0",
+      "version": "1.18.1",
       "hasInstallScript": true,
       "dependencies": {
         "better-sqlite3": "^12.11.1",