Use vendor/nim-sds as the default source for desktop builds. Add a
platform sentinel that clears shared qrcodegen, nim-sds, and libstatus
outputs when switching desktop/mobile or arch in the same tree.
Replace parse-time platform cleanup with an order-only Make target.
Drop ios/android branches from the root Makefile; mobile cleanup lives
in mobile/Makefile. Always delegate status-go mobile builds via FORCE
and copy libraries only on change.