Feature: Expose `query_stream` in Python SDK for streaming retrieval events

[zTgx]

Description

The Rust API provides Engine::query_stream() which returns a RetrieveEventReceiver that yields real-time retrieval events (navigation progress, evidence collection, sufficiency checks). This is not exposed in the Python SDK.

Python users have no way to monitor retrieval progress — the query() call blocks until complete.

Expected Behavior

# Desired API
async for event in engine.query_stream(ctx):
    if event.type == "evidence_collected":
        print(f"Found: {event.node_title}")
    elif event.type == "worker_round":
        print(f"Round {event.round}: {event.command}")