With a shape layer's Gradient Fill or Gradient Stroke (or a property in
it) selected, its Start Point and End Point show in the viewer as
draggable crosshairs with a guide between them, as in After Effects. The
points are in the space of the shape group holding the gradient, so they
are mapped through the enclosing groups' transforms and the layer's
(shapes::item_matrix), and a drag converts back the same way. A drag is
one prop.set merge: one undo step, and on an animated point a keyframe at
the current time. The hook runs after the viewer's own gestures, like the
effect point controls, so a handle takes the press and the layer doesn't
move. They follow View ▸ Show Layer Controls.
Tests (ui_gradient_handles, only pre-existing APIs, all three fail without
this): dragging the End Point moves it by the drag in one undo step without
moving the layer; inside a moved, 200%-scaled group the handle sits where
the group draws the point and a drag converts to group space; no handles
unless the gradient or a property in it is selected, or with layer
controls off. ui_effect_controls still passes.
