add .tar.tstamp files to year and top levels

These `.tar.tstamp` files will be used, along with the entry
`.tar.tstamp` files, to form compressed tarballs that
can be reproduced exactly the same way, down to the byte,
under both macOS and Linux.

Stay tuned ... :-)

2001/.filelist

@@ -7,4 +7,5 @@
 2001/mkentry.c
 2001/README.md
 2001/rules.txt
+2001/.tar.tstamp
 2001/.year

2001/.tar.tstamp

@@ -0,0 +1 @@
+1711171755