Anchoring rewrite

s/viewable/visible/

Prevent annotation attempts when not signed in

Simply hides the annotation form--which implied you could
use the thing...--and instead tells the user to sign in,
so they can do what they came to do.

While transforming a selector to a range must occur synchronously,
the individual strategies can be attempted in separate animation
frames.

Also, make this more beautiful.

All of the anchoring types are actually synchronous, for now. Having
promise callbacks between the selector->anchor and anchor->range
conversions sometimes creates race conditions when multiple
annotations anchor concurrently. Punt on this for now.

And hide the cache details from the default implementation that
doesn't use them.

I'm not sure this works because I'm not sure the first then function
will be called synchronously by the render frame invoking resolve.

This makes it more robust against change by storing its description
at the container level rather than a DOM Range object that could get
mutated unpredictably by text node splitting.

Rather than searching PDF pages in order, use the position anchor
(when available) to spiral the search outward from the page that
likely contains the quote.

Rather than searching all the pages in parallel, search them serially
and terminate early with a match.

This is maybe too verbose for now but at least it's explicit and
doesn't assume that anchoring implementations can be fully
responsible for cache invalidation.