Add column width sharing across grids

[wieslawsoltes]

Summary

Adds first-class DataGrid column-width sharing across multiple grids, analogous to Grid.SharedSizeGroup/scope usage.

• introduces an explicit weak-reference DataGridColumnWidthSharingScope
• introduces DataGridColumnWidthSharing.Group for direct XAML columns
• adds DataGrid.ColumnWidthSharingScope and DataGridColumnDefinition.WidthSharingGroup
• synchronizes initial widths and propagates automatic, programmatic, user, inherited, and redistribution changes
• tracks collection, group, scope, measurement, and visual-attachment lifecycle transitions
• documents compiled-binding/MVVM and direct XAML usage

Design

The implementation is instance-scoped with no global registry. A reentrancy guard prevents recursive broadcasts while widths are applied through the existing no-callback API.

Sizing behavior is explicit:

• pixel columns remain pixel columns;
• intrinsic auto modes retain their mode while sharing desired/display width;
• measured star results normalize to pixels because different grids cannot reliably share one star result;
• unresolved stars are deferred entirely—including their provisional constraints—until measurement completes;
• overlapping constraints produce a common clamped width, while incompatible constraints clamp each participant independently.

Definition changes use the live materialization pipeline, so group updates do not rebuild the grid.

Review hardening

Follow-up fixes cover:

• inherited width preservation;
• registration refresh and identity-safe scope migration while detached;
• active-width adoption on reattach, preventing stale detached widths from winning;
• rejection of reports from pruned/nonparticipants;
• first-measure reporting for deferred stars;
• indirect non-star/star redistribution, normalization, and min/max side-effect reports;
• consistent exclusion of unresolved stars from both target assignment and common constraint aggregation.

Validation

• Focused width-sharing suite: 17 passed, 0 failed.
• Full Avalonia.Controls.DataGrid.UnitTests: 2,267 passed, 0 failed.
• Release leak suite: 34 passed, 0 failed.
• git diff --check: clean.
• All review threads resolved.

Fixes #295

[wieslawsoltes]

Self-review completed. I reviewed scope registration, weak-reference cleanup, group changes, constraint handling, width-mode preservation, and tests; 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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