Global setModelRole marked the whole modelRoles path modified and
saveNow overwrote the freshly re-read disk record with the stale
in-memory map, clobbering concurrent/external per-role edits despite
the re-read intended to preserve them. Track modified global roles
individually and merge only those into the re-read file, mirroring the
project save path.
Fixes#6260
roboomp
14a8b709cc