Flexbox: apply transferred min/max sizes to flex base size and hypothetical sizes

[nicoburns]

Objective

Fix flexbox sizing of items with an aspect ratio and min/max constraints in the opposite axis (e.g. replaced images with max-height in a row flex container). Previously, transferred sizes were baked directly into FlexItem::min_size/max_size via maybe_apply_aspect_ratio, so they incorrectly participated in resolving flexible lengths and clamping the item's final size. Per w3c/csswg-drafts#10997 and browser behaviour, transferred sizes should only clamp:

• the flex base size / hypothetical main size,
• the content-based part of the automatic minimum size (§4.5 transferred size suggestion / content size suggestion cap),
• the hypothetical cross size.

Pseudo-diff of the approach:

 FlexItem {
-    min_size: style.min_size.resolve(..).maybe_apply_aspect_ratio(ar),
-    max_size: style.max_size.resolve(..).maybe_apply_aspect_ratio(ar),
+    min_size: style.min_size.resolve(..),   // untransferred
+    max_size: style.max_size.resolve(..),
+    aspect_ratio: ar,                       // stored; transfer applied at use sites
 }

In determine_flex_base_size and determine_hypothetical_cross_size, transferred_{min,max}_size = child.{min,max}_size.maybe_apply_aspect_ratio(child.aspect_ratio) are now used for: the cross-axis available-space clamp, the known-cross-dimension clamp, the automatic-minimum-size cap, and the hypothetical main/cross size clamps. Elsewhere (flexible length resolution, final clamping) the untransferred min/max sizes are used, as before this change they were the transferred ones.

Context

Found while fixing css-flexbox WPT failures in Blitz (companion PR: DioxusLabs/blitz#526). With this change plus the Blitz-side fixes, the following WPT tests newly pass: flex-minimum-width-flex-items-012, flexbox-min-width-auto-002c, flexbox-min-height-auto-002c, flex-minimum-height-flex-items-021, flex-aspect-ratio-img-column-005, aspect-ratio-transferred-max-size — with no regressions across the full css-flexbox suite (699 → 705 whole tests, 1765 → 1771 subtests).

All existing Taffy tests pass (cargo test: 4421 generated fixture tests green).

Feedback wanted

Taffy currently applies aspect ratios to border-box sizes; a fully content-box-aware transfer (needed for items with asymmetric borders, e.g. WPT flex-aspect-ratio-img-row-005 subtest 3) is left for follow-up.

Link to Devin session: https://app.devin.ai/sessions/0f8547b7fbb949e28e9acdbedb1ca8d2
Requested by: @nicoburns

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