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/klammerset/decl.k

@@ -0,0 +1,11 @@
+# Engine-tier fixture: a klammerset with metadata, a dependency, an ordered
+# file list (klammers.k uses the target defined in base.k), and a trailing
+# definition that must load after the files (program order).
+@@@klammerset kit | Engine-test klammerset
+  :name Klammerset Integration Test
+  :author Klammertext tests
+  :date 2026-07-30
+  :requires util.k
+  :files base.k / klammers.k
+@@@
+@@after : AFTER @@