Reconcile realized rows after collapsing all groups

[wieslawsoltes]

Summary

• reconcile the virtualized display list once after CollapseAllGroups() finishes updating every group and collapsed-slot range
• recycle any leaf rows left outside the final visible slot set before scrollbar computation and arrange invalidation
• add an 850-row, two-level grouping regression with a dominant first group and many second-level headers

Root cause

CollapseAllGroups() collapses headers one at a time through EnsureRowGroupVisibility and UpdateRowGroupVisibility. The collapse path intentionally performs incremental removals from the circular realized-element buffer with updateSlotInformation: false, then updates _collapsedSlotsTable afterward. A per-group UpdateDisplayedRows reconciliation only occurs for some scroll-anchor positions.

For distribution-sensitive multi-level grouping, those incremental operations can leave a viewport-sized run of realized DataGridRow controls outside the final visible slot set. The group model and collapsed-slot table are already correct, which is why a later scroll, resize, or expansion repairs the display, but CollapseAllGroups() previously had no final display reconciliation of its own.

Fix

After every group has been collapsed and the collapsed-slot table has settled, CollapseAllGroups() performs one final realization rebuild. For legacy scrolling it derives the first visible header and partial-row offset from the adjusted scrollbar offset; logical scrolling continues through its anchor-alignment path. The reconciliation runs once per explicit collapse-all gesture and adds no work to regular scrolling.

Regression coverage

The new headless test creates:

• 850 rows;
• a 640-row dominant first top-level group;
• dense second-level segments spanning the viewport;
• a constrained 320-pixel grid viewport.

It performs collapse → expand → collapse to exercise display-list reuse, confirms rows are realized after expansion, and then asserts immediately after the final CollapseAllGroups() call that no visible leaf DataGridRow remains.

Validation

• focused regression: 1 passed
• complete group expand/collapse suite: 5 passed
• complete Avalonia.Controls.DataGrid.UnitTests headless suite: 2,251 passed, 0 failed
• production build (net8.0, net10.0): 0 errors; existing repository warnings only
• git diff --check: clean

Closes #309

Review hardening

Follow-up review expanded the final reconciliation beyond stale-row removal:

• top-clamped collapses rebuild from the first visible group header with offset and partial-row state reset;
• positive legacy offsets are clamped to the collapsed visible extent and mapped back to the corresponding visible header/partial offset before realization;
• a regression with 80 preceding top-level groups covers collapsing while scrolled near the end of a dominant group.

Current validation: 6/6 focused collapse/expand tests and 2,252/2,252 full unit tests pass. Review threads are resolved.

[wieslawsoltes]

Self-review completed. I reviewed the collapse-all batch reconciliation point, realized-row cleanup behavior, viewport-bounded work, and regression coverage; no additional actionable findings were found. All six CI checks are successful, the branch is synchronized, and git diff --check is clean. This PR is ready for review.

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