A run reads the project, edits it and writes the whole file back, so two
at once silently dropped the edits of whichever saved first. A run that
saves over the file it opened (--save, or --save-as the same file) now
holds an OS lock on `.<name>.lock` next to the project from before its
read until it exits. The next run says so on stderr, waits, then edits
what the first one saved. Ported from filmcraft-cli
(storytold/filmcraft#3926).
The OS drops the lock however the process ends, so a crash leaves nothing
stale. If the lock can't be taken the run warns and saves as it did
before. A missing project stops the run before it opens anything, with
the same error opening gives, so a project created in between is never
saved without the lock. The desktop app and `mcp` don't take this lock.
