render: z-index paint layers and clear ending float zones

Paint order was pure tree order, so a dropdown/overlay declared before a
later sibling lost to it and a z:-1 decorative backdrop covered its own
content. A positioned element with a non-zero z-index now lifts its
whole subtree into a separate paint layer: negative layers under the
normal flow, positive above it, each sorted ascending with tree order
preserved within equal z. Nested z roots paint inside their ancestor
root's subtree in tree order, the pragmatic core of CSS stacking
contexts.

clear (left/right/both, previously unparsed) now ends the float zone,
so a clearfix element drops below the float at full width instead of
joining the flow column beside it.

Repro-verified: an earlier z:10 overlay paints over a later sibling, a
later z:-1 backdrop paints under earlier content, and the clear repro
matches Chromium within 3px on every measure. Spot set unchanged or
slightly improved; obstacle course 33/33.

crates/obscura-render/src/lib.rs

@@ -325,6 +325,13 @@ pub struct LayoutStyle {
     /// cells inherit it); obscura applies it as main-axis alignment of the
     /// cell's flex-column stand-in. `None` on non-cell elements.
     pub vertical_align: Option<VerticalAlign>,
+    /// `z-index` on a positioned element. `None` is `auto` (tree order). A
+    /// non-zero value lifts the element's whole subtree into a separate paint
+    /// layer: negatives under the normal flow, positives above it, sorted.
+    pub z_index: Option<i32>,
+    /// `clear`, when set: this element moves below preceding floats on the
+    /// given side(s), ending their float zone.
+    pub clear: Option<Clear>,
     /// Resolved during the inheritance pass: true when this element should
     /// not be painted at all, either from its own or an inherited
     /// `visibility: hidden`, or because the product of its own and every
@@ -463,6 +470,15 @@ pub enum VerticalAlign {
     Bottom,
 }
 
+/// `clear`: which floated side(s) an element moves below. Ends the float
+/// zone in `dom::build_children_with_float_zone` (the clearfix idiom).
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Clear {
+    Left,
+    Right,
+    Both,
+}
+
 /// `line-height`: `normal` (a font-relative default), a unitless multiple of
 /// font-size, or an absolute pixel length.
 #[derive(Debug, Clone, Copy, PartialEq)]