Fix flexbox aspect-ratio min sizing and floated flex container sizing

[nicoburns]

Objective

Fix layout regressions observed in DioxusLabs/blitz#625 (bumping blitz's taffy dependency from 0.12.x to git main), diagnosed via WPT css/css-flexbox/flexbox-min-height-auto-002b.html.

Four fixes:

1. Flexbox automatic minimum size (src/compute/flexbox.rs): the content size suggestion is now clamped by min/max cross sizes transferred through the aspect ratio (per css-sizing-3 §5.1), not just the transferred max size:

   let clamped_min_content_size = min_content_main_size
       .maybe_clamp(transferred_min_size.main(dir), transferred_max_size.main(dir))  // new
       .maybe_min(child.size.main(dir))
       .maybe_min(transferred_max_size.main(dir));

   Regression from Flexbox: apply transferred min/max sizes to flex base size and hypothetical sizes #989 (bisected to 0972b84): an item like <img style="height: 100px; min-width: 30px"> (aspect-ratio 1) in a 1px-high column container resolved its automatic min-height to ~0 and collapsed instead of flooring at 30px.

2. Flexbox used cross size (src/compute/flexbox.rs): when an item's cross size is transferred from its main size through its aspect ratio (cross size property is auto), determine_used_cross_size now re-derives it from the used (post-flexing) main size instead of using the hypothetical cross size that was transferred from the flex base size before flexing. E.g. after the item above is shrunk from 100px to 30px main size, its cross size is now 30px rather than 100px. The hypothetical cross size (which feeds the container's intrinsic cross sizing) is unchanged, matching Chrome.

3. Floated flex/grid containers are now shrink-to-fit sized (src/compute/block.rs): the float layout path passed AvailableSpace::Definite(available_width) with no known width. Block containers implement fit-content sizing internally in that situation, but flex/grid containers treat definite available space as stretch-fit, so a floated flex container's items were stretched to the full available width. The float path now computes the fit-content width (available.clamp(min_content, max_content) clamped by the float's min/max width) up front and passes it as a known dimension.

4. Cross-axis available space is clamped by the item's own margins, not the container's (src/compute/flexbox.rs): determine_flex_base_size/determine_hypothetical_cross_size added constants.margin (the container's margin) to the item's min/max cross sizes when clamping the cross-axis available space. A container margin (e.g. margin-right: 2px) could inflate a stretched item's cross size beyond the container's own width. This dates back to Fix: available space in the presence of a min width #407 but was latent until floats/definite available space exercised it.

Context

Found while investigating flexbox regressions in DioxusLabs/blitz#625. With these fixes plus a small blitz-side fix, both regressed WPT tests (flexbox-min-height-auto-002b.html, flexible-box-float.html) pass again.

Includes gentest fixtures with Chrome-generated expectations: aspect_ratio_flex_column_min_height_auto_transferred_min_width, float_flex_container_shrink_to_fit, and float_flex_container_max_width_margin_stretch_item. cargo test --workspace, fmt, and clippy pass.

Feedback wanted

• For fix 4, Chrome additionally floors a stretched aspect-ratio item's main size via the transferred size suggestion from its definite (stretched) cross size (e.g. stretch width 34 + aspect-ratio: 1 gives height 34 where taffy gives 30). That behavior is not implemented here; the WPT test passes in blitz because images take blitz's replaced-element path.

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/40e9a29868044e77a6dbf77a7b4b11e3
Requested by: @nicoburns

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