T25d Generative: sound and motion

AudioGenerationPlayer shows a generated sound's prompt, model and
length, a bar while it is made, then chat::AudioMessage to play it;
Regenerate and Download wait for it. TTSVoicePicker lists voices with a
sample to play, the chosen one washed with a check, tags dropping below
when narrow. VideoGenerationTimeline lays shots along their time under
a ruler, with posters, placeholders, a playhead and Extend.

AudioMessage's waveform is one Tab stop where Left and Right seek, and
it gives way at 280px. debug::ruler serves the timeline's ruler. A mask
shown to read no longer follows the pointer.

src/generative/tests/mod.rs

@@ -5,6 +5,7 @@ use crate::{forms, primitives::FocusNext, theme::Theme};
 mod canvas;
 mod results;
 mod settings;
+mod sound;
 
 fn settle(cx: &mut VisualTestContext) {
     cx.run_until_parked();