fix: rabbit review 001 - persist Streamlit failures, guard Jupyter lifecycle

- st_demo: persist failure in call history, clear active_handle,
  and disable Run while call active to avoid overwriting handle
- JupyterApp.shutdown: gather all shutdowns with return_exceptions=True
  and re-raise as BaseExceptionGroup so one failure doesn't leak kernels
- messages: fallback known-but-malformed messages to UnknownJupyterMessage
  via ValidationError, make Unknown channel permissive (str) for nuance
- transport: record reader failures as terminal state (_reader_failure)
  and fail fast from execute/messages_for instead of hanging
- shell: don't reset execution_count to 0 when IPython returns None
- README: use ipython_shell imports

Skipped non-urgent/deferred items: line(), get_shell_2, test renames,
import laziness, call-ID bounding, settle window config (worth doing later).

README.md

@@ -3,8 +3,7 @@
 The application-facing API is:
 
 ```python
-from ipython_demo import App
-from ipython_demo import CallOptions
+from ipython_shell import App, CallOptions
 
 app = App()
 call = app.run_code(