LAST LIGHT 0.1.0

The off-grid library, packaged for the Tiiny App Farm. Ask a question in plain
words, get an answer built only out of the books on the device, with the book
and the page printed under it. When the shelf does not cover the question it
says so instead of inventing an answer.

This is the books half of the archiver repo. The notes half ships separately as
tiiny-brain and is not affected by this tag.

What is in the farm build:

• ask and cite, browse the shelves, look a subject up by name
• no ingestion. Turning a PDF into text needs poppler, reaching poppler means
  starting another program, and the farm's archive scanner refuses a tree that
  can do that. The desk tool in the same repo still ingests.
• a prebuilt shelf instead: downloaded once, resumable, checked by length and
  then by hash before anything is unpacked, and unpacked through a filter that
  refuses absolute paths, parent traversal and links.
• with no shelf yet it serves and says so, and offers to fetch one.
• Python standard library only.

The shelf URL is still a placeholder. Nothing has been published to fetch, so
--download-shelf refuses and says why rather than reaching for something that
is not there.

python3 last-light --serve [--port N]
python3 last-light --selfcheck
python3 last-light --shelf