labelled this w/ nova.astrometry.net + opus 5.5

previously, visual/wysiwyg mode already had live collaboration; every change made by any collaborator was a transaction that was synced to you in real time, and vice versa, carets included. however, source mode was not a part of this: it instead kept a local mdx draft that synced on a debounce, so any invalid changes blocked the entire page, other people's changes could only sync when you stopped typing, and you also couldn't see each other's cursors. now, every edit in source mode becomes a transaction on the same shared document in visual mode. this is achieved by mapping every character of mdx to its corresponding spot in the visual document, re-parsing only parts of the document that you are editing, holding syntactically incorrect changes locally, and fitting any collaborative edits around those changes. the mapping also allows carets to be shown collaboratively across formats!