design.mps.protan is missing from the masters

[rptb1]

design.mps.protan is referenced at e.g.

mps/code/protan.c

Lines 45 to 48 in 9fd0577

	/* ProtSync -- synchronize protection settings with hardware
	*
	* <design/protan#.fun.sync>.
	*/

but is not present in https://github.com/Ravenbrook/mps/tree/master/design .

Discovered in conversation with @kbrowli about integrating MPS with Emacs on Cygwin.

[rptb1]

design/protan.txt was removed by @gareth-rees in Perforce changelist 187450 but this change did not update any references in protan.c .

The references need to be fixed up. I also suggest rewriting them in tag notation since that's more durable.