Build typed IPython app execution pipeline

README.md

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+# IPython demo
+
+The application-facing API is:
+
+```python
+from ipython_demo import App
+
+app = App()
+call = app.run_code(
+    "print('hello'); 2 + 2",
+    shell_name="new",
+    collapsed=False,
+)
+```
+
+`call` contains the submitted `code`, `call_id`, `shell_name`, `collapsed`
+state, a typed `result`, and ordered typed `events`. Event sequences are
+stable within a call, so a frontend can use `(call_id, sequence)` as its
+idempotency and React key pair.
+
+For MIME output, retain the complete bundle and choose a renderer in the UI.
+The initial preference is `text/html`, JavaScript, `text/latex`, then
+`text/plain`, with image MIME types handled by their image renderers. HTML
+must be sanitized, and JavaScript should be treated as untrusted unless the
+host explicitly opts into a sandboxed execution policy.
+
+Run the JSON showcase with:
+
+```text
+uv run ipython-demo
+```