scrollback files should be moved from the current style, scrolback/<target>.txt, to scrollback/<target>/YYYY/MM.NN. YYYY/MM are only used to choose the file being written to, and should be chosen clientside. the client should never write to a previously-written file, if it has already started on a new one.

NN is used to split when capacity is reached for an individual file. this capacity should be user-editable!

for now, until robust scrollback rendering is implemented, the procedure for loading scrollback should be to walk back through scrollback files until capacity is reached or all files have been read, then append lines from each, starting at the start of, or an offset of, the last-read file.

if there exists an old-style scrollback/<target>.txt, it should be broken up into new-style scrollback files and deleted. if there exists both an old-style file and a new-style directory, care should be taken to interleave the old into the new before deleting it. this should only be done during the call to scrollback_load.