refactor(window): collapse 3 NSHostingControllers to 1 via NavigationSplitView

[datlechin]

Summary

Architectural cleanup: replace MainSplitViewController (NSSplitViewController + 3 separate NSHostingControllers for sidebar/detail/inspector) with a single NSHostingController<MainEditorRootView> whose body is NavigationSplitView { sidebar } detail: { detail } .inspector(...). The single SwiftUI rendering root replaces three independent ones.

This is option β from the perf refactor planning thread. Ships in one PR. Window/tab/route management (TabWindowController + EditorWindow + WindowManager) is intentionally untouched — that's the WindowGroup migration which is a separate, larger, runtime-test-required PR.

What changes

Deleted:

• MainSplitViewController.swift (~497 lines) — NSSplitViewController shell with 3 split items
• SidebarContainerViewController.swift (~132 lines) — NSViewController wrapper that hosted an NSSearchField above an NSHostingController
• InspectorVisibilityProxy protocol uses (now redundant)

Added:

• MainEditorWindowState.swift — @Observable class that owns currentSession, sessionState, rightPanelState, inspectorPresented, sidebarColumnVisibility, windowTitle. Replaces the state-owning portion of MainSplitViewController. Handles connectionStatusDidChange observation.
• MainEditorRootView.swift — SwiftUI root. NavigationSplitView { sidebar } detail: { MainContentView } .inspector(isPresented:). Sidebar search uses .searchable(text:placement:.sidebar) — replaces SidebarContainerViewController's NSSearchField.

Modified:

• TabWindowController.swift — instantiates MainEditorWindowState + NSHostingController<MainEditorRootView> instead of MainSplitViewController. Owns the MainWindowToolbar directly (was previously installed by the split VC). Observes windowState.sessionState so the toolbar can be installed when a delayed connection completes.
• MainContentCoordinator.swift — inspectorProxy: InspectorVisibilityProxy? and splitViewController: MainSplitViewController? weak refs replaced with editorWindowState: MainEditorWindowState?. Inspector toggle, sidebar tab switch, etc. all flow through this object.
• MainWindowToolbar.swift — coordinator.splitViewController?.setSidebarTab(...) becomes coordinator.editorWindowState?.setSidebarTab(...). NSSplitView KVO observer (which depended on the deleted MainSplitViewController.splitView) replaced by adding _ = coordinator.editorWindowState?.sidebarColumnVisibility to the existing withObservationTracking loop. Cleaner — observation framework instead of NSNotification fan-out.
• MainContentView.swift / MainContentCommandActions.swift / MainContentView+Setup.swift — updated 3 call sites to use the new editorWindowState API.

Net code: -202 lines (522 added, 724 deleted across 10 files).

Why this is the right scope

Per WWDC 2020 "Use SwiftUI with AppKit" and Apple's apps (Xcode, Pages, Numbers): the canonical pattern is NSWindowController + a single NSHostingController containing the entire SwiftUI tree. Multiple NSHostingControllers per window is a known performance footgun — each is its own SwiftUI rendering root with independent ViewGraph, AttributeGraph, layout engine, and AppKit constraint negotiation.

The previous architecture had 3 hosting roots (sidebar + detail + inspector) plus 10 more in MainWindowToolbar's items. Each root re-evaluated its body and negotiated intrinsic sizes through NSHostingView.SizeConstraints.update(from:) independently. The 1.71s connect-window-open spike captured in the Time Profiler trace had _layoutSubtreeWithOldSize cascading 6 levels deep, with SizeFittingLayoutComputer.Engine.sizeThatFits and NSAttributedString.MetricsCache.metrics as leaves.

Collapsing the 3 layout-region hosting roots into one means:

• One ViewGraph, one AttributeGraph for sidebar+detail+inspector layout
• One pass through _layoutSubtreeWithOldSize instead of three
• One intrinsic-size negotiation, settled by NavigationSplitView's column layout

The 10 toolbar hosting roots stay for now (NSToolbar.delegate-driven items can't directly become SwiftUI ToolbarContent while the window is hosted via NSHostingController — SwiftUI's .toolbar modifier needs scene context). Their migration is part of the WindowGroup PR.

Test plan (caller responsibility)

⚠️ This PR rewrites a substantial part of the window architecture. I cannot runtime-test in the development conversation. The build and lint pass; runtime testing falls to the merger.

Critical paths to verify:

• Cold launch + Welcome → click Connect: window appears, sidebar populates with table list, data grid loads
• Connection from dock context menu (Open Recent): same
• Sidebar collapse/expand via toolbar button + Cmd+Ctrl+S
• Switch sidebar tab (Tables ↔ Favorites) via toolbar buttons
• Sidebar search: type in the search field at the top of the sidebar, table list filters
• Switch to Favorites tab: search field placeholder changes to "Filter favorites", filters favorites
• Inspector toggle via toolbar / Cmd+Option+I / menu: panel slides in/out
• Inspector content: column inspector + query result summary + AI chat tabs
• auto-show inspector on row selection (when dataGrid.autoShowInspector is on)
• Cmd+W on last tab clears to "No tabs open" via commandActions.closeTab() (EditorWindow override unchanged)
• Multi-window: open a second connection, both windows independent
• Native macOS tab grouping (same connection windows tab together) still works
• State restoration: app remembers inspectorPresented and sidebar column visibility across launches

If any of these break, follow-up fix PRs are expected.

Build & lint

• xcodebuild Debug arm64: BUILD SUCCEEDED
• swiftlint --strict on all 8 changed files: 0 violations
• No em-dashes / AI-vibe phrases in added lines

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