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)

sks/sks.k

@@ -1,15 +1,10 @@
 # Standard Klammer Set
-@read kutil/kutil.k @
-@read target/target.k @
-@read font/font.k @
-@read section/section.k @
-@read image/image.k @
-@read code/code.k @
-@read list/list.k @
-@read link/link.k @
-@read table/table.k @
-@read date/date.k @
-@read block/block.k @
-@read color/color.k @
-@read document/document.k @
-@read book/book.k @
+@@@klammerset sks | Document production: formatting, structure, and layout for the html, tex, pdf, and txt targets
+  :name Standard Klammer Set
+  :author Andy Kopra
+  :date 2026-07-30
+  :files kutil/kutil.k target/target.k font/font.k section/section.k
+         image/image.k code/code.k list/list.k link/link.k
+         table/table.k date/date.k block/block.k color/color.k
+         document/document.k book/book.k
+@@@