Add cui layout: container tree with rects and overflow markers #5

Context

When content overflows its container there is no way to see why. ui.RectOf(id) only works for widgets with an id, containers rarely have one, and a screenshot only shows the clipped result. Fitting 12 ranking rows into a 404 px card took three rebuild-and-screenshot cycles to tune paddings.

Proposal

cui layout (or cui tree --rects) prints the container tree of the last frame with each node's rectangle, scroll extent, and a flag for content larger than its box:

Card      0,0    404x780
  Scroll  16,60  372x700   content 372x1120   OVERFLOW
    Row   16,60  372x46

• Include widget kind, id when it has one, x/y/w/h, and available/clipped deltas.
• Optional in-app --debug-layout: tint boxes whose children overflow, so a single screenshot can answer the same question for visual issues (overlap, zero height, off-screen).

Acceptance

One cui layout call identifies the overflowing node and its bounds without taking an image.