fix(core): nested tab frames stay loaded and never outrank the selected tab - #11446

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What is the current behavior?

A Frame hosted as TabViewItem.view (per-tab navigation stacks) can lose its navigation bookkeeping through two separate mechanisms, both reproduced deterministically in apps/toolbox (new nested-tab-frame pages) on the iPhone simulator and a Pixel 6a API 35 emulator:

1. TabView.items re-assigned with an array holding the same items (what a reactive renderer does on every update). onItemsChanged removed and re-added every item, which unloaded and tore down the selected item's view. On iOS the re-added item's Frame was never loaded again, because TabViewItem.canBeLoaded is only set after the load cascade has already run, so frame.isLoaded stayed false while the tab was on screen and every navigate() sat in the queue until an app called frame.callLoaded(). On Android the frame's native view was recreated detached from its tab fragment. On both platforms the selected tab went blank.
2. Android preloads the neighbouring tabs (androidOffscreenTabLimit defaults to 1), so their frames load and navigate while hidden, and navigate() pushed them to the top of the frame stack. Right after startup Frame.topmost() returned the off-screen Demos frame instead of the selected Home frame, and after selecting a tab it returned that tab's hidden neighbour. Frame.topmost().navigate() pushed a page into the invisible tab, and the hardware back button (Frame.goBack(Frame.topmost(window))) would have acted on it too.

What is the new behavior?

• TabViewBase.onItemsChanged keeps items present in both arrays attached; only removed items are torn down and only new ones added (the Android override resets tab specs only for removed items). iOS additionally loads the selected item's view once its controller exists, so an item added to a loaded TabView at the selected index is not left unloaded.
• ViewBase gains _isChildPresented(child); TabViewBase overrides it for the selected item. FrameBase._pushInFrameStack() only promotes a frame that every ancestor presents; a hidden frame is kept at the bottom of the stack (still found by Frame.getFrameById) until its tab is selected, when the existing _pushInFrameStackRecursive() call promotes it.

Verified on device after the change: the reassignment keeps isLoaded=true and pushes keep working on both platforms; on Android Frame.topmost() is the selected tab's frame at startup and after every selection, with the preloaded neighbour parked below (stack=[Third, root, Home, Demos] after selecting Demos).

Unit tests: tab-view-common.spec.ts (retained/removed/added items, item moved between TabViews, _isChildPresented) and new frame stack presentation cases in frame-common.spec.ts (hidden frames stay off the top, remain findable by id, preloaded tab frame navigating does not outrank the selected tab, selection promotes it).

Fixes #11444.