Support last-baseline alignment (align-items: last baseline) in flexbox and grid

[nicoburns]

Objective

Implement last-baseline alignment (CSS align-items: last baseline / align-self: last baseline) in flexbox and grid layout, on top of the last-baseline output support in #1110 (this PR is based on that branch and should merge after it).

Context

Final PR of the last-baseline series (#1107 → #1108/#1109 → #1110 → this).

Style API (src/style/alignment.rs):

• New AlignItemsKeyword::LastBaseline variant and AlignItems::LAST_BASELINE constant (serde name "LastBaseline").
• CSS parsing accepts last baseline (and first baseline as an alias of baseline); safe/unsafe modifiers remain invalid with baseline keywords.

Flexbox (src/compute/flexbox.rs):

• Items with align-self: last baseline form a second baseline group, separate from first-baseline items, using the same participation rules (no auto cross-axis margins, per flexbox §8.3).
• calculate_children_base_lines now measures each group independently (skipping groups with ≤1 member), storing FlexItem::last_baseline with the same scroll-container border-box clamping as first baselines; missing baselines are synthesized from the border box.
• Line cross-size calculation accounts for last-baseline groups via child.last_baseline + max_last_baseline_descent.
• Cross-axis alignment anchors the last-baseline group to the cross-end of the line (mirroring how the first-baseline group anchors to cross-start), with wrap-reverse flipping as for first baselines:

  AlignItemsKeyword::LastBaseline => if constants.is_wrap_reverse {
      max_last_baseline - child.last_baseline
  } else {
      line_cross_size - max_last_baseline_descent - child.last_baseline
  }

• In columns (where horizontal baselines can't be aligned along the cross axis), items synthesize baselines from their border boxes with the group anchored to the line's cross-end edge, matching Chrome's behaviour (verified via the generated align_last_baseline_column fixtures, including RTL).
• The container's own last baseline now prefers items participating in last-baseline alignment (mirroring the first-baseline logic).
• Absolutely positioned items treat last baseline like end for static position purposes.

Grid (src/compute/grid/):

• GridItem gains a last_baseline_shim which acts as an extra bottom margin (the mirror image of the existing top-margin baseline_shim), applied both during intrinsic track sizing (margins_axis_sums_with_baseline_shims) and final alignment (align_and_position_item now takes a Line<f32> of top/bottom shims).
• Track sizing computes shims per row for each baseline group independently; last-baseline shims are derived from each participating item's last-baseline descent (distance from its last baseline to the bottom of its margin box).
• For positioning, LastBaseline falls back to End alignment within the grid area, which combined with the bottom shim aligns the items' last baselines.

Tests: 9 new gentest fixtures (36 generated tests) covering flex row/column/wrap-reverse/mixed-groups/auto-margin-exclusion and grid basic/nested/mixed-groups cases, plus parser and serde round-trip tests. cargo fmt, clippy, and the full test suite pass.

Feedback wanted

• During track sizing, GridItem::last_baseline transiently stores the item's last-baseline descent rather than the baseline itself (documented on the field); it is overwritten with the real baseline from the item's final layout before the container's last baseline is computed. A separate field could be used instead if this overloading is deemed too subtle.
• Flex column fallback behaviour for last baseline (left edges aligned, group anchored to the line's cross-end) was reverse-engineered from Chrome; other interpretations of the spec's fallback-alignment rules exist.

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/c0a80d9344d54e918b37cacfb71a2128
Requested by: @nicoburns

[staging-devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR that start with 'DevinAI' or '@devin'.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment, CI, and merge conflict monitoring