docs(video): subtitle-driven demo script with a generator

The video now carries subtitles instead of a voice-over, because the words are
better written once and checked than spoken under time pressure in a second
language.

demo-subtitles.json holds one cue list with the timing, the English line that
goes in the video, the Korean line for whoever is editing, and what belongs on
screen. scripts/make-srt.mjs writes both .srt files from it, so the two
languages cannot drift apart, and refuses to write a file whose lines run past
42 characters (English) or 24 (Korean), or that ask a viewer to read faster than
17 and 12 characters per second.

32 cues, exactly 3:00, no warnings. The shot list is rebuilt around them, with
recording steps for the tools already on Windows 11, and the QR beats added.