MOAD-0002 (Intertangle): CLEAN

darktable uses a global `darktable` struct that holds all subsystem pointers
(darktable.db, darktable.undo, darktable.develop, darktable.pwstorage, etc.).
This is intentional architecture: darktable is a single-process application
with tight subsystem coupling by design. The subsystems do not independently
evolve and share a single GTK main thread. No accidental Intertangle coupling
between independently-deployable subsystems found.
