Form sections hold their children (win32, UIKit); win32 paints gradients, orders zstacks, keeps a tooltipped widget's text

[nicolas-maman]

The styled example on Windows showed its title and its buttons and nothing between. Five defects behind that, all fixed and verified on screen.

1. section()'s children went into the title label (win32 and UIKit). The DSL returns handle + 1 as the box children go into — the inner container GTK4 and AppKit register right after the frame. These two backends registered the title there, so every field and toggle of a section was parented to a label. The inner box is registered first now (then the title, which is still shown first).
2. win32 did not treat forms, sections and navstacks as stacks. They are create_stack windows with a kind of their own, and every "is this a stack" test in the layout listed the three plain kinds: a form measured as 0×0 (its parent read the window's current size), requested no layout and did not show. One predicate, w32_is_stack, at every site; measure_subtree goes by orientation rather than kind.
3. style_bg_gradient was stored and never painted on win32. The container's erase paints it (GradientFill), and a label, spacer or rule inside paints the slice of the gradient under itself (w32_erase_ground; WM_CTLCOLOR answers with a hollow brush after painting the slice), so a caption on a gradient header is text over the header rather than a box of one colour.
4. win32 zstack: the last child is on top, margins hold, and the ground behind is the panel's. add_child pushes every new child to the bottom of the Z order (right for a row or a column, so enumeration reads in creation order), which in a zstack put the label added after the panel under it, with the two painting over each other in whichever order Windows chose. Children are raised in creation (handle) order and clip their lower siblings; a child's margins inset it (margin_of over a panel, as on GTK4's overlay); w32_ground_owner looks through a zstack's lower siblings so a label over a coloured panel paints the panel's colour behind its text.
5. style_tooltip wiped the widget's text on win32. set_tooltip freed the text cache, so every button with a tooltip reported "" to the driver and to the a11y name fallback.

Verified natively: runtime test; 76 headless driver specs (the three that fail on this box are the GL-less gpuview_demo and the AETHER_UI_CHROME/AETHER_UI_TOOLTIP env-gated tumbling_cube/vg_tooltip, as before); 14 on-screen specs; captures of the styled example and a gradient/zstack probe.