Nested (no urlOwner), the shell's pick is claimed with its depth, and the owner above records it as
this shell's ?subtab= segment. First paint claims the current tab as a replace, so the segment exists
before a deeper one; a link that names a row arriving with the manifest is still claimed only once the
row is known. The shell reads its tab from its own segment first, then ?tab= only when it names one of
its rows (a one-level partner page), on load and on Back/Forward. Each child is told its depth: 1 under
an owning shell, one deeper otherwise. Pages where the shell owns the address are unchanged.
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