The surface-specific half of the binding.
Figure id and title, the include-list, per-worksheet overrides, and the live-update opt-in.
A binding, or null when there was nothing to mount — no worksheet
to read, nowhere to mount, or worksheets that yielded no navigable layer. In
every null case the page is left exactly as it was found, with one warning
explaining why.
Read the host's worksheets, mount MAIDR for them, and keep it in step.
The host must already be readable: waiting for
firstinteractive, or forinitializeAsync, is the binder's job, since only it knows which one applies.