fix(ui): force stdlib asyncio policy so 3-D Twin tab works under uvloop

[harsh-pandhe]

Summary

`make ui` crashed every time the 3-D Twin tab tried to render:

```
Process Process-N:
...
ValueError: Can't patch loop of type <class 'uvloop.Loop'>
```

Root cause

uvicorn ships uvloop as a transitive dependency of Streamlit. If the parent Streamlit process's event-loop policy is uvloop's, `multiprocessing.Process` (forked by stpyvista's trame backend to render the 3-D scene) inherits it. The subprocess then calls `asyncio.run` → `nest_asyncio2._patch_loop`, which only knows how to patch stdlib loops, and bails on `uvloop.Loop`.

Fix

• Reset `asyncio.set_event_loop_policy(asyncio.DefaultEventLoopPolicy())` at app import time. Streamlit's tornado server is unaffected; the child process now spawns a stdlib loop that `nest_asyncio2` can patch.
• Wrap the `stpyvista(...)` call in try/except so any future backend crash degrades to a clear warning + `st.exception` panel rather than a silent subprocess traceback. The rest of the tab (sliders, compliance badges, GeoJSON import) stays interactive.

Test plan

• `make ui` no longer prints the uvloop traceback in the terminal
• 3-D Twin tab renders the interactive PyVista scene
• No other code paths touched; the 157-test suite is unaffected

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