3207 - simplify make output for demo

There's this tradeoff between adding complexity to the makefile and
adding some complexity to its output, and I've been writing for myself.
Experimenting with sliding just a tad toward simpler output for others.
3208 - pass tangle final file rather than all files
3209 - new script: build until a layer
3210
3211
3212
3213 - layers 0-9 build separately

Includes get duplicated now, but it's easier to show the key property:
that excluding layers should still result in a working program.