refactor: move UI-test harnesses out of AppDelegate into extension files

[arzafran]

What this does

AppDelegate.swift had grown to 14,818 lines, and roughly a fifth of that was code that only exists to support XCUITest automation (cmd-click diagnostics, stress-workspace loaders, jump-to-unread/goto-split/bonsplit-drag/multi-window-notification test harnesses). None of it runs in production builds. This moves that instrumentation into three same-type extension files so AppDelegate.swift itself is smaller and easier to navigate, with zero behavior change.

Summary

• Sources/AppDelegate+UITestCmdClick.swift (726 lines): cmd-click terminal UI test setup, socket sanity check, display-resolution diagnostics. Moved as a single #if DEBUG block (lines 3103–3815 of the original file).
• Sources/AppDelegate+UITestStressWorkspaces.swift (547 lines): debug-stress-workspace loaders (openDebugStressWorkspacesWithLoadedSurfaces and its helpers). Functions only — the stored properties they depend on stay in the AppDelegate class body (Swift doesn't allow stored properties in extensions), re-wrapped in their own #if DEBUG.
• Sources/AppDelegate+UITestHarnesses.swift (1,803 lines): jump-unread, goto-split, bonsplit tab-drag, and multi-window-notification test harnesses. Moved as a single #if DEBUG block.
• Sources/AppDelegate.swift: 14,818 → 11,783 lines. All moved code deleted; no call sites changed.
• GhosttyTabs.xcodeproj/project.pbxproj: registered the 3 new files (fileRef + buildFile entries, group membership, Sources build phase) mirroring AppDelegate.swift's existing entries.

Visibility widened (private/fileprivate → internal only — no other change) because the split moved code across file boundaries within the same type:

• MainWindowContext (private final class) → internal
• Members defined outside the moved clusters, now called from moved code: writeUITestDiagnosticsIfNeeded, socketListenerConfigurationIfEnabled, restartSocketListenerIfEnabled, debugManagerToken, sendTextWhenReady, plus stored properties the compiler flagged during the build pass: mainWindowContexts, didSetupJumpUnreadUITest, jumpUnreadFocusExpectation, jumpUnreadFocusObserver, didSetupTerminalCmdClickUITest, didSetupGotoSplitUITest, didSetupBonsplitTabDragUITest, terminalCmdClickUITestPoller, bonsplitTabDragUITestRecorder, gotoSplitUITestRecorder, gotoSplitUITestObservers, didSetupMultiWindowNotificationsUITest, didSetupDisplayResolutionUITestDiagnostics, displayResolutionUITestObservers, ghosttyGotoSplitLeftShortcut/RightShortcut/UpShortcut/DownShortcut, debugStressWorkspaceCreationInProgress, debugStressLagProbeEnabled, debugStressWorkspaceCount, debugStressPaneCount, debugStressTabsPerPane, debugStressYieldInterval, debugStressSurfaceLoadTimeoutSeconds, debugPerfWorkspaceTitlePrefix.
• Members defined inside the moved clusters, now called from code that stays in AppDelegate.swift: the six setup...UITestIfNeeded entry points (setupJumpUnreadUITestIfNeeded, setupTerminalCmdClickUITestIfNeeded, setupGotoSplitUITestIfNeeded, setupBonsplitTabDragUITestIfNeeded, setupMultiWindowNotificationsUITestIfNeeded, setupDisplayResolutionUITestDiagnosticsIfNeeded), plus logSlowShortcutMonitorLatencyIfNeeded, recordGotoSplitMoveIfNeeded, recordGotoSplitSplitIfNeeded, recordGotoSplitZoomIfNeeded, recordMultiWindowNotificationFocusIfNeeded, scheduleUITestSocketSanityCheckIfNeeded, writeJumpUnreadTestData, writeMultiWindowNotificationTestData.

Nuclear-review N3/AS2.

Test Plan

• Local Debug build green: PROGRAMA_SKIP_ZIG_BUILD=1 xcodebuild -project GhosttyTabs.xcodeproj -scheme programa -configuration Debug -destination 'platform=macOS' build → ** BUILD SUCCEEDED **
• CI arbiter — this is a pure move with no logic changes, so the key checks are ui-regressions and the UITest-driven jobs (jump-unread, goto-split, bonsplit-drag, multi-window-notification, cmd-click) passing unchanged.