After purchasing a genuine DEC VT340 terminal, @hackerb9 has learned that the DEC VT340 graphics programmer's manual was mistaken and the sense of DECSDM should be inverted. Nearly every terminal emulator which built upon that reference over the last thirty years is incorrect. Some are correcting the error now, some never were wrong, but we are at a time when it is not safe to presume what setting DECSDM means.
For now, lsix will simply presume that sixel scrolling is enabled and hope that no program has been run before it that disables it.
See #41 for details.
Thanks go to @j4james for pointing out the mistake and for testing multiple terminal emulators to show that none of them currently default to having sixel scrolling disabled.