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

env/runtime.env

@@ -42,6 +42,12 @@ unset _kt_tl _kt_arch
 # --- PATH (TeX Live appended only if one was found) ---------------------------
 export PATH="$KLAMMERTEXT_HOME/bin:$KLAMMERTEXT_HOME/tst${KLAMMERTEXT_TEXLIVE_BIN:+:$KLAMMERTEXT_TEXLIVE_BIN}:$PATH"
 
+# --- The Python binding (kpy/) -----------------------------------------------
+# "import klammertext" with no setup, as the container image already arranges
+# through its own PYTHONPATH.  Prepended, so a source build shadows any other
+# copy; the module is bound to the Python the library was built against.
+export PYTHONPATH="$KLAMMERTEXT_HOME/kpy${PYTHONPATH:+:$PYTHONPATH}"
+
 # --- Per-platform: LSan suppressions + shared library search path -------------
 # The shared library path deliberately includes ONLY Klammertext's own .so dirs,
 # NOT third-party bundled-library dirs. In particular NVIDIA iray's platforms/