A recent update to the X Windowing System (or whatever it’s called) once again changed the names of its monitors / displays / output devices, so that my startup script no longer confined the tablet to the landscape display.
In mostly reverse chronological order, here are various commands I’ve puzzled out:
#xsetwacom --verbose set "HUION Huion Tablet stylus" MapToOutput "DP1-8"
xsetwacom --verbose set "HUION Huion Tablet stylus" MapToOutput "DP-1-8"
#xsetwacom --verbose set "HUION Huion Tablet Pen stylus" MapToOutput "DP-1"
#xsetwacom --verbose set "Wacom Graphire3 6x8 Pen stylus" MapToOutput "DP-1"
#xsetwacom --verbose set "Wacom Graphire3 6x8 Pen stylus" MapToOutput "HEAD-0"
#xsetwacom --verbose set "Wacom Graphire3 6x8 Pen eraser" MapToOutput "DP-1"
#xsetwacom --verbose set "Wacom Graphire3 6x8 Pen eraser" MapToOutput "HEAD-0"
Over the last two years, the display name changed from DP-1 to DP-1-8 to DP1-8, and back to DP-1-8. I grew accustomed to this with the Wacom tablet (HEAD-0‽)and now know where to look, but I still have no idea of the motivation.
Aaaand the tablet’s stylus name? The Wacom names were stable, but the Huion names apparently come from the Department of Redundancy Department.
The Smell of Molten Projects in the Morning 
Not for nothing is it said that there are only two hard problems in computer science: cache invalidation, naming things, and off-by-one errors.
There is nothing quite like watching an off-by-one error in CNC programming … [sigh]