The Python binding (kpy/): klammertext.Machine from a script

Assembled from klammertext-dev ed13dfaa5b7a.  kpy/klammertext.cpp is a
CPython extension module giving a Python script an instance of the
Machine class -- ktext in-process, set up through the same functions the
command uses.  Errors map one-to-one to Python exceptions under the C++
names; an @eval's Python exception is chained as __cause__.  Built by
make -C mac, found through PYTHONPATH (env/runtime.env sets it), tested
by tst/kpy_test.sh.  Three engine changes came with it: Error::message(),
set_command_state(), and the @eval error report carrying its traceback.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Uv7KWxafinDPraswEf4GsC

mac/error.cpp

@@ -1,3 +1,5 @@
+#include <sstream>
+
 #include "error.h"
 #include "show.h"
 #include "util.h"
@@ -13,26 +15,33 @@ bool display_source(const std::string& filename_arg)
     }
 }
 
-void Error::print_message(const std::string& epilog)
+std::string Error::message(const std::string& epilog) const
 {
     // The separator is not cosmetic: the description ends with a sentence, and
     // appending the advice straight onto it ran the two together into one word
     // ("...for an unspecified targetkdiag loads no klammerset...").  m_desc is
     // then justified to 80 columns, so the break has to be a blank line rather
     // than a single newline, which justify() would fold back into the flow.
+    std::string desc = m_desc;
     if (epilog != "") {
-        m_desc += "\n\n" + epilog + "\n";
+        desc += "\n\n" + epilog + "\n";
     }
-    m_desc = justify(m_desc, 80, 0);
-    std::cerr << "\n" << red << command_name << " (" << m_type << " error)";
+    std::stringstream ss {};
+    ss << command_name << " (" << m_type << " error)";
     if (display_source(m_loc.m_filename)) {
-        std::cerr << ":\n  " << m_loc.m_filename;
+        ss << ":\n  " << m_loc.m_filename;
     }
     if (m_loc.m_line > -1) {
-        std::cerr << ", line " << m_loc.m_line;
+        ss << ", line " << m_loc.m_line;
     }
     if (m_loc.m_chr > -1) {
-        std::cerr << ", character " << m_loc.m_chr + 1;
+        ss << ", character " << m_loc.m_chr + 1;
     }
-    std::cerr << ":\n\n" << m_desc << reset << "\n";
+    ss << ":\n\n" << justify(desc, 80, 0);
+    return ss.str();
+}
+
+void Error::print_message(const std::string& epilog)
+{
+    std::cerr << "\n" << red << message(epilog) << reset << "\n";
 }