refactor(datagrid): cell-based sort indicators with native divider behavior

[datlechin]

Summary

Architectural refactor on top of #942 / #944. Replaces the overlay NSImageView indicator approach with native NSTableHeaderCell-based drawing. Side benefits: cleaner code, working hover cursor on dividers, no spurious sort on divider taps.

• Sort indicators drawn in NSTableHeaderCell: New SortableHeaderCell overrides drawInterior to render the system ascending/descending arrow at the right edge, with a small priority number to its left for secondary sort columns (Numbers-style). SuppressedSortIndicatorCell deleted.
• Header view simplified: SortableHeaderView no longer manages overlay subviews. updateSortIndicators now pushes state into each column's SortableHeaderCell and triggers a per-column redraw.
• Divider tap → resize, not sort: mouseDown detects clicks within the column-resize zone and routes them straight to super.mouseDown, so AppKit's column-resize handling runs and no sort cycle fires for taps on the divider.
• Hover cursor on dividers: mouseMoved sets NSCursor.resizeLeftRight when the pointer is in a resize zone, NSCursor.arrow otherwise. Backed by a .mouseMoved tracking area and a resetCursorRects registration.

Test plan

• Hover a column divider in a table tab → cursor changes to ↔.
• Click a divider without dragging → no sort triggered.
• Click-drag a divider → column resizes.
• Click a column header (away from divider) → sort cycles asc → desc → cleared.
• Shift-click a second column → multi-sort, primary shows arrow, secondary shows 2 ↑ style indicator.
• Shift-click descending secondary column → removed from sort.
• Drag a column header to reorder → reorder works.
• Right-click a column header → context menu appears.
• `xcodebuild test` — `HeaderSortCycleTests` and `SortableHeaderResizeZoneTests` pass.
• `swiftlint lint --strict` — 0 violations.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Restore window mouse-move policy when header leaves window

viewDidMoveToWindow unconditionally sets window?.acceptsMouseMovedEvents = true, but this commit never restores the previous value when the header view is removed or replaced. Because acceptsMouseMovedEvents is a window-wide switch (default false), any window that ever hosts this header will continue dispatching mouse-moved events globally, which can introduce unnecessary event traffic and side effects in other views after the grid is gone.

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