Grid Lanes: stacking-axis content distribution and self-alignment

[nicoburns]

Objective

Stacking-axis alignment for Grid Lanes (css-grid-3 §6.3 and §6.4), stacked on #1274. Baselines are out of scope: baseline alignment in the stacking axis falls back to start, which is what the spec does (§6.5).

• Content distribution (§6.3): align-content/justify-content in the stacking axis moves the stacking range as a whole. center and the distributed values fall back to center/start per the spec, end packs to the end edge, normal/stretch behave as start, and safe clamps the free space at zero.
• Self alignment (§6.4): an item adjacent to a gap (the last item in a track, or one followed by a spanning item) aligns within its margin box plus that gap. The gap is the free space after the item's margin box and gutter up to the next item in every spanned track, or the content box end for the last item. Items that are not last in all of their tracks have no alignment space, which is what Chromium does (GetAvailableAlignmentSpaceForItem). Dense-packed items take the space below them in the opening and cut the space of the item above.
• normal self-alignment behaves as start in the stacking axis (Chrome 151 does not stretch auto-sized items by default); stretch stretches items without an explicit size into the gap.

All code is in the feature-gated lanes.rs; the shared grid code is untouched. 14 new Chrome 151 fixtures (56 tests) in test_fixtures/grid_lanes/ cover content distribution (center/end/safe/space-around), self-alignment (end/center/stretch/normal/align-self, with and without a stacking gutter), both directions, rtl and dense packing. All 204 lanes tests pass; the existing suites regenerate byte-identically.

Context

• Placement records, per item, an alignment_space (LanePlacement) and, per track, the item placed last (last_in_track). Placing an item in a track closes the previous item's space in that track; after the container's stacking-axis size is known, the last items get the space up to the content box end. The item's stacking-axis area passed to align_and_position_item is its margin box plus that space, so the existing self-alignment code handles start/center/end/stretch.
• For dense packing, Opening now carries the item placed before the skipped space (item_above), as Chromium's TrackOpening::alignment_candidate does, so backfilling adjusts both items' spaces.
• Stacking-axis stretch/normal is overridden to start on the lanes GridItems: GridItem::known_dimensions would otherwise return the stretched size as the item's contribution during placement (the container's stacking-axis size is the percentage basis). Positioning re-reads the styles, so stretch still applies there.
• Chrome 151 behaviour confirmed with the fixtures: the last item in a track aligns up to the container's content-box end; a spanning item that is no longer last in one of its tracks does not move.

Feedback wanted

• Spec §6.4 says the last items' alignment container ends at the "lowest bottom outer edge" among last items when the container is indefinite; with a definite stacking-axis size Chrome uses the content box end, which is what this PR does in both cases (with an indefinite size the two coincide).

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

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

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