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_registry.h

@@ -0,0 +1,63 @@
+#pragma once
+#include <map>
+#include <optional>
+#include <string>
+
+#include "klammerset.h"
+#include "argument_set.h"
+#include "katom.h"
+
+class Klammerset_registry
+{
+public:
+    Klammerset_registry();
+
+    void add(Klammerset klammerset);
+    // Parse a @@@klammerset span.  Returns the new Klammerset so the
+    // Machine can load its files, or nullopt when the symbol is already
+    // registered: a klammerset is loaded once, and a repeated declaration
+    // (typically reached through :requires) is skipped, not an error.
+    std::optional<Klammerset> add(std::vector<Katom>::iterator begin, std::vector<Katom>::iterator end, std::vector<Katom>& katoms);
+    void check_symbol(const std::string& symbol, const Locator& loc) const;
+
+    bool has(const std::string& symbol) const;
+    Klammerset get(const std::string& symbol, const Locator& loc) const;
+
+    std::string describe(int margin=2, bool long_format=false) const;
+
+    std::map<std::string, Klammerset> m_klammersets {};
+    std::vector<std::string> m_symbols {};
+    Parameter_set m_parameters {};
+};
+
+// --- The klammerset search path (symbol -> declaration file) ---
+
+// A bare identifier (letters, digits, underscores; starts with a letter)
+// names a klammerset symbol; anything else -- a path component, an
+// extension, a space -- is a filename used as given.
+bool is_klammerset_symbol(const std::string& name);
+
+// The directories searched for a klammerset symbol, most specific first:
+//   1. local_dir -- the directory of whatever names the symbol: the input
+//      document's directory (ktext -k), the declaring file's directory
+//      (:requires), or the cwd when there is no document
+//   2. the KLAMMERTEXT_KLAMMERSETS directories (colon-separated; default
+//      ~/.klammertext/klammersets), in listed order
+//   3. $KLAMMERTEXT_HOME (the distribution's sets, e.g. sks/sks.k)
+// A symbol x resolves to the first <dir>/x/x.k found, so a document-local
+// set shadows an installed one, which shadows a distributed one.  (This is
+// deliberately the OPPOSITE order from the @eval module path, which puts
+// the SKS directories first: a stray util.py next to a document shadowing
+// an SKS module is an everyday accident, while a directory named sks/
+// holding an sks.k next to a manuscript is not.)
+std::vector<std::string> klammerset_search_dirs(const std::string& local_dir);
+
+// Resolve a symbol to its declaration file; first hit wins.  Throws a
+// Klammerset_error naming the searched directories when nothing matches.
+fs::path resolve_klammerset_symbol(
+    const std::string& symbol, const std::string& local_dir, const Locator& loc);
+
+// The symbols available on the search path, with provenance; a symbol
+// found again in a later directory is marked as shadowed.  For
+// kdesc --klammerset.
+std::string describe_klammerset_search(const std::string& local_dir, int margin=2);