Document language for dates; @eval pathname resolution and :cwd (from dev bc7cd62b6f68)

@date/@datetime gain :lang (German: "16. Juni 1910") over a document-wide
Language state variable, and :number for the numeric form (en 6/16/1910,
de 16.06.1910 per DIN 5008).  @eval finds Python modules next to the file
that names them regardless of the cwd, and the new :cwd option runs an
eval in a chosen working directory (@source_file uses it to resolve
against the document).  Two new test suites ship in tst/.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

mac/state.h

@@ -77,10 +77,16 @@ public:
 
     void parse_state_katoms(std::vector<Katom>::iterator begin, std::vector<Katom>::iterator end, katom_list katoms);
     std::vector<std::string> all_names();
+    void add_search_dir(const std::string& dir);
     std::string python_code();
     std::string describe(bool show_environment=false, int margin_size=2) const;
 
     std::vector<Frame> m_frames {};
+    // Directories of the files the Machine has read (input files, klammer
+    // sets, @read targets), in reading order: @eval finds Python modules
+    // and :cpp libraries next to the file that uses them (see python_code()
+    // and Eval::eval_command).
+    std::vector<std::string> m_search_dirs {};
     // @@@state Image_search_path :set :append :replace :argtype :desc
     // Parameter_set m_parameters = Parameter_set("name :set :append :replace :argtype :delim :desc");
     Parameter_set m_parameters = Parameter_set("name :value :append :replace :delim :desc");