Bug: Engine.__init__ creates a separate tokio runtime, conflicting with async environments #92

Description

PyEngine::new() in python/src/engine.rs:116 creates its own Runtime::new() and calls rt.block_on() to build the engine. This creates a separate tokio runtime from the one used by pyo3_async_runtimes for async method calls.

This can cause issues when:

1. The user is already running an async event loop (e.g. Jupyter, FastAPI)
2. Multiple Engine instances are created — each spawns its own runtime
3. The background graph rebuild task runs on a different runtime than the engine's async methods

Relevant Code

• python/src/engine.rs:116-143 — PyEngine::new() with Runtime::new().block_on()

Suggested Fix

Use pyo3_async_runtimes::tokio::get_runtime() to share a single tokio runtime, or lazily initialize the engine on first async method call instead of in __init__.