Grid Lanes: dense packing

[nicoburns]

Objective

Third Grid Lanes PR: dense packing (css-grid-3 §4.3 and §4.4 step 4). Stacked on #1272.

After an item's normal placement and measurement, a dense container backfills it into the highest skipped space it fits into, provided the tracks of that space have the same total size as the tracks of its normal placement (so the measurement stays valid). Among spaces within the tie threshold of the highest one the start-most wins. A backfilled item leaves the auto-placement cursor and the running positions untouched; definitely placed items can backfill within their own tracks.

The dense keyword is read from grid_auto_flow().is_dense(): the spec has not settled which property carries it (csswg-drafts#12803), Chromium uses a provisional grid-lanes-pack property, and Blitz drives it from grid-auto-flow: dense on a display: grid-lanes container (DioxusLabs/blitz#1132). The fixtures set both properties so Chrome and Taffy agree.

Context

Implementation in src/compute/grid/lanes.rs, following Chromium's shape (GridLanesRunningPositions):

• TrackOpenings per track (only allocated when dense): bounded skipped spaces plus the open end at the running position. Normal placement records a skipped space whenever a spanned track's running position is below the item's position; backfilling carves the item out of the opening (split, trim or remove).
• find_dense_placement tries every start line with the same spanned track size and intersects the openings across the span (lowest_opening_in_span, recursive like Chromium's AccumulateTrackOpeningsToAccommodateItem); paths that end in the open end of every track are not skipped spaces. Two passes (highest, then first within the threshold that is above the normal placement) avoid a candidate allocation.
• Items occupy outer size + stacking gap in the openings, matching the running-position bookkeeping.
• set_item_grid_axis_lines is the previous inline line-setting code, now shared by normal and dense placement.

Tests: 9 new fixtures in test_fixtures/grid_lanes/grid_lanes_dense_*.html (36 generated tests) covering backfill, the same-track-size rule, spanning backfill across adjacent openings, definite items, the tie threshold, gaps, margins, row direction and the no-opening fallback; all pass against Chrome 151 and the existing suites regenerated byte-identical. cargo test --all-features: 6869 passed.

No shared grid code changes: everything is in the grid_lanes-gated lanes.rs, so default-feature grid layout is unaffected.

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/93f06aa5dd4e4bc18d2482197dbb9191
Open in Devin Desktop: https://dioxus.staging.devinenterprise.com/desktop/session/93f06aa5dd4e4bc18d2482197dbb9191?variant=devin-insiders
Requested by: @nicoburns

[staging-devin-ai-integration]

I'll fix CI failures and address comments from users with write access that start with 'Devin'.

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