chore: cite docs/BUGS.md and drop references to notes outside the repo

66 comments cited `bugs.md B<n>`, the ledger's old name; they now say
`docs/BUGS.md B<n>`, and every cited number has an entry. About 90 more
cited offline.md, devices.md, features.md, mount-architecture.md,
plan.md and improvements.md, planning notes that were never in the
repository. Those citations are removed; the comments keep their text.

crates/pdfs-fuse/src/shutdown.rs

@@ -6,7 +6,7 @@
 //! invisible; in an in-process remount — which is how the daemon recovers from a
 //! mount going away underneath it — it leaks a whole generation of threads that
 //! keep writing to the database and calling the API on behalf of a mount that no
-//! longer exists (bugs.md B44).
+//! longer exists (`docs/BUGS.md` B44).
 //!
 //! The primitive is deliberately small: a flag plus a condvar. Every loop that
 //! would otherwise call `thread::sleep` calls [`Shutdown::sleep`] instead, which