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)

mac/klammerset.h

@@ -0,0 +1,28 @@
+#pragma once
+
+#include <string>
+#include <vector>
+
+#include "locator.h"
+
+// A Klammerset is the formal construct declared by the @@@klammerset system
+// command: a named, logically related group of klammer definitions loaded
+// together.  The instance holds metadata and the file list only -- the
+// klammers themselves live in the Machine's Klammer_registry (one flat,
+// additive namespace; the definition modes govern collisions).  Membership
+// is provenance, not containment.
+class Klammerset
+{
+public:
+    Klammerset() = default;
+    Klammerset(const std::string& symbol, const std::string& desc, const Locator& loc);
+
+    std::string m_symbol {};   // registry key and command-line name (e.g. "sks")
+    std::string m_desc {};     // the logical relationship that makes this a set
+    std::string m_name {};     // display name (e.g. "Standard Klammer Set")
+    std::string m_author {};
+    std::string m_date {};     // last update; a date serves as the version
+    std::vector<std::string> m_requires {};  // klammerset declaration files, loaded first
+    std::vector<std::string> m_files {};     // definition files, loaded in list order
+    Locator m_loc {};          // the declaring file; relative names resolve against it
+};