feat: stack layout with sizing rules #87

What

Layout is columns and a vertical cursor, held as loose fields on the context (ox, oy, cx, cy, pw) that each container saves and restores by hand. begin_panel does not save them at all, so a panel inside a window overwrites the window's layout. Toolbars, forms and panels also need horizontal stacks, alignment and sizing rules without hand-computed coordinates.

A single-pass immediate-mode layout cannot know a container's content size before placing it, so fit-content sizing and center or end alignment read the size measured last frame from the state store (#95) and settle one frame after content changes, as Dear ImGui's auto-fit does.

Acceptance

• A layout stack on the context that every container (panel, window, popup, dock, scroll region, columns) pushes and pops, replacing the loose cursor fields, with a test nesting a panel in a window
• Horizontal and vertical stacks that nest, each with spacing and alignment (start, center, end, stretch)
• Sizing per item: fixed, fit content, fill, and a fraction of the remaining space, with min and max
• Same-line placement, as ImGui's SameLine, for quick inline rows
• Layout reads its gaps and padding from the style system
• Existing column helpers keep working on top of the new layout, so current callers do not change