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/command.h

@@ -20,3 +20,18 @@ parse_args(
     std::string output_basename, bool display_only);
 
 void load_klammersets(Machine& machine, const strings_t& symbols);
+
+// The state a command gives a Machine before anything is read: the frame
+// named for the command and the K_* variables that describe the run --
+// target, output directory and basename, whether output is displayed rather
+// than written, the input filenames, the verbosity, and the input directory
+// (the first input file's, or the cwd when there is none).  This was the top
+// of ktext's main(), and it is policy, not mechanism: a klammer set reads
+// these names (@document reads K_stdout_only and K_output_dir), so a second
+// consumer of the Machine -- the Python binding in kpy/ -- must set exactly
+// the same ones or a document that renders under ktext fails under it.  One
+// function, two callers, nothing restated.
+void set_command_state(
+    Machine& machine, const std::string& frame_name, const std::string& target,
+    const std::vector<std::string>& input_filenames, const std::string& output_dir,
+    const std::string& output_basename, bool display_only);