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/font_store.h

@@ -19,7 +19,7 @@ struct Resolved_font {
     float capheight_ratio = 0.0f;  // cap-height / unitsPerEm from OS/2 table
 };
 
-std::string name_to_dirname(std::string name);
+std::string name_to_dirname(const std::string& name);
 // Directories searched for installed fonts (KLAMMERTEXT_FONTS, default
 // ~/.klammertext/fonts); the default font set is searched after them.
 std::vector<std::string> installed_font_dirs();
@@ -27,7 +27,7 @@ std::vector<std::string> installed_font_dirs();
 std::string default_font_dir();
 // Every installable font family found across those directories.
 std::vector<std::string> available_font_families();
-Resolved_font resolve_font(std::string family_name);
+Resolved_font resolve_font(const std::string& family_name);
 void install_resolved_font(const Resolved_font& font, std::string output_dir);
 
 // One font file classified by its internal metadata (name table, OS/2).