T25c Generative: canvas

InpaintCanvas lays a mask over a picture: brush strokes in shares of
the picture, rastered into a 320-column grid where a later stroke wins
and an erasing one clears, painted as merged rectangles snapped to
device pixels. A ring follows the pointer at the brush's size; [ and ]
size the brush and X swaps it when focused. MaskBrush sets paint or
erase and the size, with Undo and Clear waiting for a stroke.

Pointer strokes move to forms::pen, which SignaturePad and the canvas
share.

AGENTS.md

@@ -81,6 +81,7 @@ Ely GPUI Component. A component library for GPUI, in light and dark.
 - A rounded box does not clip its children. Each layer inside takes the radius itself; `checker` takes one.
 - A window keeps one `feedback::Toaster` and renders one `ToastViewport`; anything holding the entity pushes to it. Messages share `primitives::Severity`.
 - Only gpui can build `ExternalPaths` with paths, so drop rules live in `forms::files::dropped`, where tests reach them.
+- A stroke drawn with the pointer goes through `forms::pen`: a press starts it, a drag extends it, and the lift hands it over, in or out of the pad (`SignaturePad`, `generative::InpaintCanvas`).
 - gpui runs key bindings before key listeners. A container takes a child's bound key through the action with `capture_action`, as `Form` takes `Submit`.
 - `ScrollHandle::scroll_to_item` runs in the container's prepaint against last frame's bounds and overflow, so a call before a list's first frame does nothing. Call it from a child's prepaint once the handle has bounds, then request one frame (`navigation::editor_tabs`). A floating list reveals its cursor, and a strip its focused item, through `forms::options::reveal` and `revealer`. A focused Tab stop deeper in a scroll box sets the box's offset whole from its own painted bounds in prepaint, once per focus (`tables::header`). Never poll layout with `request_animation_frame`.
 - gpui animations run on the wall clock; timers run on the executor's clock, which tests advance by hand. A test that needs an animation settled turns on the theme's reduced motion. Clocks read `background_executor().now()` and wake only their own view.