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.h

@@ -17,6 +17,14 @@ public:
         , m_loc(locator)
         {}
 
+    // The report text as the commands print it -- the header naming the
+    // command and the error kind, the location when there is one, and the
+    // description justified to 80 columns -- WITHOUT colour or the
+    // surrounding newlines.  print_message() writes exactly this to stderr;
+    // the Python binding (kpy/) makes it the exception's str(), which is
+    // why the two are one function: an error reads the same from a script
+    // as from ktext.  const: the former print_message mutated m_desc.
+    std::string message(const std::string& epilog="") const;
     void print_message(const std::string& epilog="");
 
     std::string m_type {};