backdraft 0.13.0

A week on saying the thing a caller would otherwise learn the hard way: what this install can do, what was withdrawn, which session a bind judged against, and when a flag was ignored.

• backdraft doctor reports every optional capability at once — the registry, page images (poppler), the vision model, legacy .xls, the entail check and math rendering — each ready or missing. A missing one gets a costs line saying what the verbs do without it, and its reason is the exact string the verb prints when it hits the gap, so the two cannot disagree. Keys are shown as set or absent, never their values. Read-only, and always exit 0. Run it first in an unfamiliar environment.
• backdraft ls --withdrawn lists what forget took out, each with the date it went and the backdraft ingest that brings it back. Plain ls is unchanged, except that a registry whose every source is withdrawn now says so instead of claiming to be empty.
• bind names the session it judged not_shown against on its first line (against session <id>), and closes with a warning when that is the shared default session. The artifact carries the same meaning: the legend's not_shown entry says what a default session costs, and backdraft verify prints a session: default line from the file alone.
• ingest --slug on a source already ingested now says --slug not applied and names the slug the document keeps. Nothing renames a document — not --slug, and not forget followed by a re-ingest, which brings it back under the same slug. The dry run says the same.
• A damaged or foreign .backdraft/registry.db is now one exit-1 line naming the file, instead of a traceback on every command; nothing is repaired or replaced, and doctor reports it as the registry's gap.

1578 tests.