Klammerset: the @@@klammerset construct, its search path, and const correctness

The @@@klammerset system command formally declares a klammerset — a
named, logically related group of klammer definitions — with an
operative, idempotent declaration (:requires and :files load in order
at the declaration point, relative to the declaring file). A bare
symbol given to ktext -k, kdesc --input, or :requires resolves to
x/x.k on the search path: the document's directory, then
KLAMMERTEXT_KLAMMERSETS, then KLAMMERTEXT_HOME; kdesc --klammerset
lists the available sets. sks/sks.k is the first declared klammerset,
so `-k sks` loads the SKS by name. The engine's lookup classes were
renamed *_set → *_registry to keep the two concepts apart, and the
whole C++ tree now follows standard const-correctness conventions.
tst/ gains klammerset_test.sh (18 cases).

(from dev 64b1abf23e56)

tst/Makefile

@@ -1,6 +1,6 @@
 # Klammertext distribution test suite (subset).
 #
-# Runs the seven shell regression suites:
+# Runs the eight 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
@@ -8,6 +8,7 @@
 #   alone_test.sh    — an optional argument's three values (default, the
 #                      argument type's :alone value, a written value)
 #   modulepath_test.sh — @eval finds modules beside the file that names them
+#   klammerset_test.sh — the @@@klammerset system command and its search path
 #   editor_test.sh   — editor support (doc/edit): indentation and table
 #                      alignment; needs python3, uses Emacs when installed
 #
@@ -21,4 +22,5 @@ test:
 	./filename_test.sh
 	./alone_test.sh
 	./modulepath_test.sh
+	./klammerset_test.sh
 	./editor_test.sh