Audit keyboard, focus, veils, motion and clocks across all 500

A cross-cutting pass, each fix with a test that failed first.

Veils: MyGo still hands a key to a base that turned off this frame,
so every element key now goes through pixel.Shortcut, which an
archtest enforces. Input handlers record and the build applies them
only while enabled; the editor turns away a veil-frame letter or
copy; value bases get a copy and keep only enabled turns. Popups
give a held focus back to their opener when the veil lifts.

Motion: S5 is enforced at the clock. Every Tick, Since, Step,
Toward and Shot runs at 4-15 fps; blinks keep 1 Hz on 4 fps steps,
fast animations step at 15, and drawn time is quantized to frames.

Clocks stepping back: Since holds at zero without overflow, and
k.Elapsed moves a stored start to now, so nothing goes negative,
runs backward or indexes out of range.

Also: logs count typed text instead of holding it, gallery names
use glyphs the font has, removable rows are keyed, components take
their own Options (archtest), and sweeps over all 508 demos check
veiled keys, clicks, wheels, the clock stepping back and names.

media/stepsequencer.go

@@ -11,7 +11,7 @@ import (
 )
 
 type StepSequencerOptions struct {
-	// Tempo is BPM, four steps a beat; zero means 120.
+	// Tempo is 1–225 BPM, four steps a beat; zero: 120.
 	Tempo    int
 	Playing  bool
 	Disabled bool
@@ -42,6 +42,10 @@ func StepSequencer(c *ui.Context, text string, tracks []string, steps [][16]bool
 		panic(fmt.Sprintf("media: StepSequencer has %d tracks and %d rows", len(tracks), len(steps)))
 	}
 	tempo := pixel.Or("StepSequencer Tempo", o.Tempo, 120)
+	// S5 caps the playhead at fifteen steps a second.
+	if tempo > 225 {
+		panic(fmt.Sprintf("media: StepSequencer Tempo %d over 225", tempo))
+	}
 	res := StepSequencerResult{Step: -1}
 	labels := 0
 	for _, t := range tracks {
@@ -59,7 +63,7 @@ func StepSequencer(c *ui.Context, text string, tracks []string, steps [][16]bool
 	// The playhead is the music; Reduce Motion keeps it.
 	if o.Playing {
 		per := time.Minute / time.Duration(4*tempo)
-		n := int(k.Now().Sub(sq.since) / per)
+		n := int(k.Elapsed(&sq.since) / per)
 		res.Step = n % 16
 		k.At(c, sq.since.Add(time.Duration(n+1)*per))
 	}