Replace MediaRecorder video export with WebCodecs + custom WebM muxer

MediaRecorder/captureStream kept crashing the Chrome renderer
(STATUS_BREAKPOINT). Video is now encoded offline with VideoEncoder
(VP9 with VP8 fallback) and muxed by a dependency-free EBML/Matroska
writer. Adds precise export controls: video fps (24/30/60), exact
length (1-8 loops or 5-60s), and a GIF loop-forever/play-once toggle.

Co-authored-by: Cursor <cursoragent@cursor.com>

index.html

@@ -89,6 +89,7 @@
 <script src="js/shaders.js"></script>
 <script src="js/engine.js"></script>
 <script src="js/gifenc.js"></script>
+<script src="js/webmmux.js"></script>
 <script src="js/exporter.js"></script>
 <script src="js/ui.js"></script>
 <script src="js/main.js"></script>