Setting the local file back as the tokens source, or unlinking the
library that was the source, stored the file's own id in
:tokens-source. ensure-tokens-lib only created a tokens-lib when
:tokens-source was nil, so a file without a tokens-lib never got one,
and every token change was dropped without any error.
Now ensure-tokens-lib also creates the tokens-lib when the source is
the file itself, which fixes files already in that state, and
set-tokens-source removes the attribute instead of storing the file's
own id.
Closes#12180
AI-assisted-by: claude-opus-5-5
31a079a7a2