fix(hig): rebuild the key view loop so Tab crosses window panes (#1490)

[datlechin]

Part of #1490 (keyboard, focus, accessibility). The cross-pane Tab item.

MainSplitViewController never called recalculateKeyViewLoop(), so the AppKit key view loop had no defined traversal across the three NSSplitViewItem panes (sidebar, detail, inspector). This calls window.recalculateKeyViewLoop() at the end of viewWillAppear, the documented mechanism for letting AppKit rebuild the geometric Tab order (leading sidebar to trailing detail to inspector). Both the sidebar List and the data-grid NSTableView are key-view-loop participants, so Tab should now move focus between panes.

This is the one item the HIG research flagged as not explicitly documented for an NSViewRepresentable/NSHostingController boundary, so it needs a real build to confirm Tab actually crosses from the SwiftUI sidebar into the representable grid. The call is harmless if a given pane does not participate (it just orders the views that do), and there is no custom focus handling.

Lint clean, style gate clean.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Recalculate the loop after materializing the inspector

When the inspector is hidden on launch, this marks the key-view loop dirty before the inspector pane contains its real controls (viewDidLoad installs Color.clear until showInspector() calls materializeInspectorIfNeeded()). If the user tabs anywhere before opening the inspector, AppKit will rebuild the loop without those inspector views; later showInspector() swaps in the inspector and expands the split item without another recalculateKeyViewLoop(), so Tab still cannot reach the inspector pane. Consider recalculating after materializing/showing panes as well as on initial appearance.

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