Proposal 0016: bb_python

[jimsynz]

Summary

Adds proposal 0016 — bb_python, a safety-aware Python integration layered over pythonx (Livebook's in-process CPython embedding).

Key shape:

• Single BB.Python.Broker GenServer serialises Python access — one GIL, one interpreter, predictable scheduling
• BB.Python.Robot Spark extension with a python do … end block carrying a pyproject.toml; uv_init runs at compile time for releases, runtime for dev
• Pre-dispatch and mid-flight safety gating, the latter via PyThreadState_SetAsyncExc raising KeyboardInterrupt to interrupt in-flight Python work
• BB.Python.PubSubBridge + an injected bb Python module exposing bb.on(name, fn), bb.publish(path, payload), bb.safety_state() — scripts cannot arm/disarm
• Marshalling for Vec3 / Quaternion / Transform (as NumPy arrays) / Localize.Unit / BB.Message, plus opaque Pythonx.Object handoff so tensors don't round-trip through Elixir
• Optional BB.Python.Action.Call and BB.Reactor.Step.PythonCall wrappers for symmetry with bb_jido and bb_reactor

Explicit non-goals: realtime control loops in Python, multiple interpreters, in-Python safety control.

Closes #17.

Test plan

• Review against pythonx v0.4.10 to confirm API references (uv_init/2, Pythonx.eval/3, Pythonx.Encoder, Pythonx.Object) match current shape
• Confirm the mid-flight safety interrupt approach (PyThreadState_SetAsyncExc) is reachable through pythonx's public surface, or scope it as a v0.2 follow-up
• Decide on the open questions (Nerves cross-compilation recipe, out-of-process broker option, bb.spawn, atom decoding policy)
• Move to implemented/ once bb_python ships