Grid: exclude items with auto block-axis margins from baseline alignment

[nicoburns]

Objective

Per css-align-3 §9.5, a box with an auto margin in the relevant axis does not participate in baseline alignment. The grid algorithm didn't check for this: the container's first-baseline selection preferred any first-row item with align-self: baseline (even with an auto block-axis margin), and such items were also baseline-shimmed during track sizing.

This is observable in WPT css/css-grid/alignment/grid-baseline-001.html sub-case "We don't participate in baseline alignment if there's an auto margin": an inline-grid with children <div>baseline</div> and <div style="align-self: baseline; margin-top: auto">below</div> should generate its baseline from the first item, but taffy picked the auto-margin item.

Context

The fix mirrors the existing flexbox implementation (FlexItem::participates_in_baseline_alignment, which checks that neither cross-axis margin is auto):

• New GridItem::participates_in_baseline_alignment():

  self.align_self == AlignSelf::BASELINE && !self.margin.top.is_auto() && !self.margin.bottom.is_auto()

• Container first-baseline selection (compute/grid/mod.rs) now picks the first first-row item that participates in baseline alignment, falling back to the row's first item in grid order
• has_baseline_aligned_item and resolve_item_baselines (track_sizing.rs) use the same predicate: only participating items are counted, have their baselines resolved, and receive a baseline_shim. Non-participating items (including auto-margin items) are no longer shimmed, so an auto-margin item is bottom-aligned by its auto margin instead

Tests: two new generated-test fixtures mirroring the WPT sub-case (grid_baseline_child_auto_margin_container_baseline, grid_align_items_baseline_child_auto_margin); all 8 generated permutations fail on main and pass with this fix. Full test suite regenerated with just gentest; cargo test --workspace, cargo fmt and cargo clippy are clean.

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/9ea93cbb618a491a9f4240b8a4780893
Requested by: @nicoburns

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