Read-mostly panel-surface contract for the overflow molecule, skin
sub-components, and future panel-level helpers. Narrower than CngxTabGroupHost - register / unregister stay on the brain.
The single write path is selectById; routing back through the
presenter preserves clamping, disabled-skip, and commit-action
gating. Template resolvers let skin sub-components project the
*cngxTabLabel / *cngxTabContent slots without depending on the
concrete CngxTabGroup class.