fix(hig): show the resize cursor over SwiftUI-hosted split dividers

[datlechin]

Fixes #1905.

Problem

The split divider on the Users and Roles tab drags to resize but never changes the pointer to the resize cursor, so it does not look draggable.

AppKit's automatic divider cursor rides on the classic cursor-rects system, which does not fire once an NSSplitView is mounted inside an NSHostingController. Every tab-content split is hosted that way, several SwiftUI layers deep under MainSplitViewController.detailHosting. The window's own sidebar and inspector dividers show the cursor only because MainSplitViewController is the window's contentViewController directly, with no SwiftUI host in between. Dragging still works because divider hit-testing is independent of cursor rects.

#1872 fixed a different layer (holding priority and fittingSize so the divider could move at all) and never touched cursors, so it left this in place. The same bug affected four SwiftUI-hosted splits, not just Users and Roles.

Fix

A shared component that every hosted split routes through, rather than a one-off patch:

• ResizeCursorSplitView: NSSplitView adds a key-window tracking area over a padded divider hit zone and sets the resize cursor in mouseMoved, the same hand-rolled approach SortableHeaderView already uses for column resize. It picks NSCursor.columnResize / rowResize on macOS 15+ and falls back to resizeLeftRight / resizeUpDown on macOS 14.
• ResizeCursorSplitViewController: NSSplitViewController installs that split view in loadView().
• CollapsingSplitViewController (Users and Roles list/detail, the privilege editor, and Structure triggers), ServerDashboardSplitView, and QuerySplitView now use it.

Divider hit-testing is a pure SplitDividerCursorGeometry helper so it is unit-tested without a live window. MainSplitViewController (native, already correct) is untouched. A new invariant in CLAUDE.md records why a hosted split needs the explicit cursor.

Testing

• swiftlint lint --strict: clean on the changed files.
• Added SplitDividerCursorGeometryTests covering vertical and horizontal dividers, three-pane splits, collapsed panes, and point hit-testing. A live cursor-image assertion is not possible in headless CI, so the pure geometry is the tested boundary.
• Manual: hover a Users and Roles divider and confirm the resize cursor appears, the divider still drags, and its position still persists.

https://claude.ai/code/session_01A5xT92pokfSFFYVjBgfnJP

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[chatgpt-codex-connector]

Avoid SDK-only cursor APIs in Xcode 15 builds

When building with the documented Xcode 15/macOS 14 SDK, NSCursor.columnResize(directions:) and rowResize(directions:) are not declared, and the #available(macOS 15.0, *) runtime check does not prevent the compiler from resolving those symbols. Since README/CONTRIBUTING still advertise Xcode 15+ support, this makes the app fail to compile for a supported build environment; use the existing resizeLeftRight/resizeUpDown APIs or hide the newer symbols behind an SDK-safe shim.

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