Captain's log for the loose-thread sweep, plus the new tool it produced.

tools/ardour-drop-missing-sources.py: Ardour's "Missing File" modal appears once
per unreadable source, no flag suppresses it, and "skip all missing" does not fix
it — Ardour substitutes silent stubs so the session loads, but the dead names stay
in the session file, so the modal returns on every launch. PLN hit it twice in one
evening. This drops the <Source> and every <Region> naming it, in <Regions> and in
all 14 playlists.

It refuses to act on a guess, twice over:
  - it will not write while Ardour is running, because Ardour holds the session in
    memory and would save over the edit on quit, silently undoing everything;
  - it will not drop a reference until it has PROVEN the file is gone from both the
    session tree and the Freebox, the SSOT for audio here. A reference is the last
    breadcrumb pointing at a lost take. And an unfinished search reports as
    unfinished rather than as "not found" — only one of those justifies dropping
    the breadcrumb.

Its first version rglobbed the Freebox once per missing file: 24 full traversals
over the network, and it never finished. Now one indexed os.walk per root against
a set of names, with the deadline checked per directory so a fruitless search
still stops on time.

041 is marked resolved in place: the blocker's premise was wrong.