fix: guard accessibility updates behind window != nil

[RoyalPineapple]

Problem

AXRuntime maintains an internal element cache (a map table) that is allocated during the scene lifecycle. Under XCTest's test runner, the scene lifecycle is delayed — the cache is still NULL when the first accessibility broadcast fires, and NSMapGet crashes on the NULL pointer.

NSMapGet (x0 = 0x0, NULL map table)
  ← AXRuntime: _AXIsInElementCache
    ← AXRuntime: __AXCreateAXUIElementWithElement
      ← UIAccessibility: _axuiElementForNotificationData
        ← UIAccessibility: _massageNotificationDataBeforePost
          ← UIAccessibility: _UIAXBroadcastMainThread
            ← libdispatch: _dispatch_main_queue_drain

Root cause

ReceiverContainerView.init sets isAccessibilityElement = true during init. When this view is added to the hierarchy via addSubview, UIKit internally posts an accessibility notification. Under XCTest's test runner, AXRuntime's element cache hasn't been allocated yet when that notification fires during a run-loop drain, crashing on the NULL map table.

We can't guard UIKit's internal notification post — the fix is to not set isAccessibilityElement = true until the view is in a window, at which point the scene lifecycle has completed and AXRuntime is initialized.

Evidence

• Crash reproduced 100% when the app is launched via xcodebuild test
• Does NOT reproduce via xcrun simctl launch (normal launch)
• Confirmed with LLDB: conditional breakpoint on NSMapGet where x0 == 0 hit the NULL map table
• Only appeared after adopting .deferredAccessibilityReceiver() on views present during initial layout

Fix

• ReceiverContainerView: move isAccessibilityElement = true from init to didMoveToWindow(). The view only becomes an accessibility element after it has a window. Sets isAccessibilityElement = false when removed from a window. Also defers needsAccessibilityUpdate = true to didMoveToWindow() so the first accessibility update doesn't trigger before the scene is ready. Removed the redundant isAccessibilityElement = true and needsAccessibilityUpdate = true from the backingViewDescription apply block since didMoveToWindow handles both.

• CombinableView.updateAccessibility(), ReceiverContainerView.updateDeferredAccessibility(), SourceContainerView.updateAccessibility(): early-return when window == nil. Defensive guards to prevent unnecessary accessibility work on views not in the hierarchy.

• Add missing accessibilityTraits override on CombinableView: the getter was missing the lazy-update pattern that all other accessibility property overrides use. Added for consistency.