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.cpp

@@ -259,6 +259,16 @@ std::vector<std::string> State::all_names()
 }
 
 
+void State::add_search_dir(const std::string& dir)
+{
+    if (!dir.empty()
+        && std::find(m_search_dirs.begin(), m_search_dirs.end(), dir)
+           == m_search_dirs.end()) {
+        m_search_dirs.push_back(dir);
+    }
+}
+
+
 std::string State::python_code()
 {
     (void)K::log(3);
@@ -267,7 +277,12 @@ std::string State::python_code()
     std::stringstream ss {};
 
     ss << "import sys\n";
-    for (auto d : sks_dirs()) {
+    strings_t python_dirs = sks_dirs();
+    // The directories of the files this Machine has read: a module next to
+    // the file whose @eval names it is found regardless of the cwd.
+    python_dirs.insert(python_dirs.end(),
+                       m_search_dirs.begin(), m_search_dirs.end());
+    for (auto d : python_dirs) {
         auto python_files = pathnames_with_extension(d, "py");
         if (!python_files.empty()) {
             ss << "sys.path.append('" << d << "')\n";