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/target_registry.h

@@ -0,0 +1,73 @@
+#pragma once
+#include <map>
+#include <string>
+
+#include "target.h"
+#include "argument_set.h"
+#include "katom.h"
+
+class Target_registry
+{
+public:
+    static std::string declare_name;
+    static std::string general_name;
+
+    Target_registry();
+
+    void add(Target target);
+    void add(std::vector<Katom>::iterator begin, std::vector<Katom>::iterator end, std::vector<Katom>& katoms);
+    void check_for_previous_definition(const std::string& name, const Locator& loc) const;
+
+
+    /*
+    void add_transforms(std::string target_name, std::string transforms);
+    void add_transforms(
+        std::string target_name,
+        std::vector<std::pair<std::string, std::string>> transforms);
+    */
+
+
+    bool has(const std::string& target_name) const;
+    Target get(const std::string& target_name, const Locator& loc) const;
+    void transform(const std::string& target_name, std::vector<Katom>& katoms) const;
+
+    std::vector<std::string> user_defined() const;
+    std::vector<std::string> applicable() const;
+
+    std::string describe(int margin=2, bool long_format=false) const;
+
+    //Argtype_registry m_argtypes {};
+    std::map<std::string, Target> m_targets {};
+    std::vector<std::string> m_names {};
+    Parameter_set m_parameters {};
+
+
+    /*
+    std::string m_parameters_spec {"name | desc :after_apply :after_write :includes | transforms.rest"};
+    Parameter_set m_parameters =
+        katomize(line_split(m_parameters_spec), Locator().str());
+    */
+
+
+    //Parameter_set m_parameters =
+    //    Parameter_set(katomize({"name :desc | transforms.rest"}, "target_registry"));
+
+    /*
+    // Targets(Statevar_set& statevars);
+
+    target_ptr check_target_existence(
+        std::string name, Locator loc, bool should_exist);
+    void add(std::string name, std::string desc, std::string include,
+    // Statevar_set& statevars,
+             std::string transform,
+             std::string before_apply, std::string after_apply, std::string after_write,
+             Locator loc);
+    target_ptr check_target_name(
+        std::string target, bool allow_all, Locator loc);
+
+    std::map<std::string, target_ptr> targets {};
+    std::vector<std::string> names {};
+    void describe_suffixes();
+    void describe();
+    */
+};