Output last baselines from all built-in layout algorithms

[nicoburns]

Objective

Compute and output the last baseline of a container (LayoutOutput::baselines.last) from all built-in layout algorithms (flexbox, grid and block), building on the Baselines { first, last } representation introduced in #1107. Last-baseline alignment (align-items: last baseline) is not part of this PR and will follow separately.

Context

Last baselines ride along with the existing layout passes — no additional layout computation is performed. Selection rules (per css-align §9.1 "baseline export" / css-flexbox §8.5):

• Flexbox: the last baseline is generated from the cross-end-most line (the first line for wrap-reverse containers), from that line's last flex item. Each item's last baseline is captured during the final layout pass, mirroring the existing first-baseline handling:

  item.baseline      = baseline_offset + baselines.first.unwrap_or(size.height)  // existing
  item.last_baseline = baseline_offset + baselines.last.unwrap_or(size.height)   // new

  with both clamped to the item's border box for scroll containers (csswg#7660).
• Grid: the last baseline is generated from the last row containing items, using the row's first item's last baseline (synthesized from its border box when it has none). align_and_position_item now returns the child's Baselines (clamped for scroll containers), stored on GridItem as last_baseline.
• Block: the container's last baseline is the last baseline of its last in-flow child that has one (the first baseline is unchanged: the first baseline of the first in-flow child with one). Both are shifted together when align-content offsets the in-flow group.

Since no built-in leaf produces baselines and nothing consumes baselines.last yet, this is only observable via custom trees / measure functions, so coverage is a new hand-written test suite (tests/hand_written/last_baseline.rs) with a minimal custom tree whose leaves report fixed first/last baselines, asserting the root LayoutOutput.baselines for flex row/column/wrap/wrap-reverse, grid multi-row, and block containers.

cargo fmt / clippy / cargo test --workspace all pass.

Feedback wanted

• Grid's first baseline currently uses the alignment baseline (item.baseline, only set for baseline-aligned items) with a border-box fallback, whereas the new last baseline uses the child's actual reported last baseline. This asymmetry preserves existing first-baseline behaviour; happy to align the first-baseline path with the "actual baseline" approach in a follow-up if desired.

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

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