SPIKE - allow users to use multiple chapters, each at different URLs, of an online textbook (like Pressbooks or Libretext)

### Problem you are trying to address with this feature
Instructors using the LMS app, wanting to use an online textbook like Pressbooks, often find they have to either assign one chapter/URL at a time, convert multiple pages to a single PDF, or else use the Hypothesis web app. In general the web app ends up being the easiest experience if the goal is to read and annotate multiple chapters/URLs as part of a single assignment,

Via LMS will open links in a new tab to try and help indicate that the new page is no longer in the LMS context. This gets tricky with pages where Hypothesis is embedded ([Pressbooks](https://pressbooks.com/), [Libretexts](https://libretexts.org/), [American YAWP](https://www.americanyawp.com/)) because the new window opening also shows Hypothesis, and not all users can tell that they're in a new app, or whether something "broke", etc.

We want to allow users to select multiple chapters of a Pressbook (or similar), or else just be able to follow links from one chapter to the next, even if we don't try to define a stopping point or range of pages.

### Your solution
We need to be able to allow users to access multiple chapters of an online resource, especially textbooks, within a single LMS assignment. This might involve joining a web app account to a 3rd-party (LMS) account, creating an LMS app extension, finding a way to remain in the LMS context when following links in Via, etc. The purpose of this Spike is to explore feasible ways forward and identify the one that we think is best.

Slack: https://hypothes-is.slack.com/archives/C01B9TRVATZ/p1732545375842969

Start by reviewing the proposed LMS assignment flow and the linked Slack discussion. Compare the options named in the issue—account linking, an LMS app extension, and preserving LMS context while following links—and document feasible approaches with a recommended path forward.

Written by the indexing model from the issue text.