Generative: a mask shown to read takes no pen; strokes fail loud

InpaintCanvas is a Tab stop with the focus ring only when on_brush
serves its keys, and takes the pen, the crosshair and the ring only
with on_stroke. new() asserts each stroke's radius within REACH and
its points on the picture. The settled strokes' cells stay cached, so
a drag paints only the stroke under the pen over a copy. The canvas
keeps square corners, so the wash fits the picture. The gallery drags
with moves.

examples/gallery/pages/generative/mod.rs

@@ -30,6 +30,7 @@ const SCRIPT: &[Step] = &[
     Step::Wait(300),
     Step::Shot("verdict"),
     Step::DownAt("gen-canvas", 300.0, 70.0),
+    Step::DragTo("gen-canvas", 390.0, 220.0),
     Step::UpAt("gen-canvas", 390.0, 220.0),
     Step::Wait(300),
     Step::Shot("masked"),