Fix display:contents transparency in box-tree construction

[UMCEKO]

Stacked on #452 (contains its commit; will rebase once it lands).

display: contents is transparent for box generation, but three places in collect_layout_children treated the contents element as opaque. Together they made a common Dioxus/Tailwind structure (a class="contents" shell between a scroll container and absolutely-positioned pages) lay out as zero-size or content-size boxes. Each fix is covered by a test in packages/blitz-html/tests/display_contents.rs:

1. Classification didn't recurse into contents children. A container whose in-flow children were all contents wrappers classified as all_inline (the existing TODO: make "all_inline" detection work in the presence of display:contents) and became an inline root, embedding hoisted block children as zero-width inline boxes. The scan now recurses through contents children; the contents node itself casts no vote — including not clearing all_out_of_flow, so a container whose only box-generating descendants are out-of-flow takes the out-of-flow path instead of becoming an inline root.

2. The DisplayInside::Contents arm hoisted the wrong generation. It called collect_layout_children on each child, which pushes the child's layout children — so leaf children contributed nothing and vanished from the layout tree. Extracted push_hoisted_children_and_pseudos, which pushes the children themselves and recurses only through nested contents nodes; the arm also gains ::before/::after support. The same helper now serves the all_out_of_flow early-return, which previously pushed the container's direct children — a contents node holding the out-of-flow elements was itself pushed as a layout box, and an absolute inset:0 child then sized against it instead of the real containing block (content-sized: a real-app page measured 1745x50517 inside a 1004x1251 scroller, leaving nothing scrollable).

3. Whitespace-only anonymous-block cleanup corrupted the child list. It removed the anon block with layout_children.pop(), assuming it was the most recent entry. Hoisted contents children can land after it, so pop() dropped a real child and left the removed anon block's stale id in the list — a slab invalid key panic in resolve_layout_children. Removal is now by identity.

WPT css/css-grid + css/css-flexbox: 1154 tests pass before and after.

[UMCEKO]

Rebased the mac ci fix