Ship the escaping test suite; add Provenance to the README

tst/ now carries all three engine regression suites (cond, deftype,
escape) and the Makefile runs them. The README gains a Provenance
section: this repository is a curated snapshot of the private
development tree, stamped with the development commit it was assembled
from.

(from dev f6463478da4c)

tst/Makefile

@@ -1,8 +1,9 @@
 # Klammertext distribution test suite (subset).
 #
-# Runs the two shell regression suites:
+# Runs the three shell regression suites:
 #   cond_test.sh    — @cond argument delimitation
 #   deftype_test.sh — the four klammer definition modes + redefinition table
+#   escape_test.sh  — target character escaping and quoted specials
 #
 # Requires KLAMMERTEXT_HOME set and `ktext` on PATH (build it with `make -C com`).
 
@@ -10,3 +11,4 @@
 test:
 	./cond_test.sh
 	./deftype_test.sh
+	./escape_test.sh