Not the owner: a rail pick announces the tab with this shell's depth (claimView),
and an outer owner such as Plugin Studio records it as its subtab segment; on a
partner's page nobody listens and nothing is written. Once the tab is settled
after the manifest (never at mount, so a link's Experimental tab is honoured),
the shell names it once with a replace, so a deeper shell's claim always has
this segment to sit under. On load it reads its own subtab segment when it names
one of its tabs, then tab when it does (a partner's page), else its default.
Back follows the segment only once the shell knows an owner records it.
Children are told their depth: 1 under an owner, else this shell's depth + 1.
As the owner (its own leaf, DX Life) the code path is unchanged.
8b34ed86dc