refactor(datagrid): drop redundant resize cursor and consolidate sort dispatch

[datlechin]

Summary

• Drop SortableHeaderView's resetCursorRects / viewDidMoveToWindow / layout / updateTrackingAreas / mouseMoved overrides. AppKit's NSTableHeaderView already adds the resize cursor at every column edge whose resizingMask.contains(.userResizingMask). Remove the resize-zone bypass at the start of mouseDown; the existing columnWidthsChanged post-check handles drag-to-resize naturally.
• Consolidate three sort delegate methods (dataGridSort + dataGridRemoveSortColumn + dataGridClearSort) into one dataGridSortStateChanged(_ state: SortState). HeaderSortCycle.nextTransition already computes the full newState; the dispatch fan-out + coordinator's three-way re-derivation was double-bookkeeping.
• MainContentCoordinator collapses handleSort + clearSort + removeMultiSortColumn into one handleSortStateChanged(_:).

Net delta: 12 files, +110 / −482 (−372 LoC). Multi-column sort, shift-click cycle (add → toggle → remove), priority-numbered chevrons, drag-to-resize, drag-to-reorder, "Don't Sort" context menu — all preserved.

Stacked on #1130. Review that one first.

Test plan

• Click column header: ascending → descending → cleared cycle
• Shift+click another column: secondary sort added, priority "2" chevron shown
• Shift+click again on same: direction toggles
• Shift+click third time: column removed from sort
• Right-click header → "Don't Sort": all sort cleared
• Hover near right edge of column header: cursor changes to horizontal resize
• Drag column edge: live resize, no sort fired afterwards
• Drag column header to reorder: column moves, no sort fired
• Multi-sort persists across tab switches (saved on QueryTab)

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