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

app/streamlit_app.py

@@ -10,8 +10,18 @@
 
 from __future__ import annotations
 
+import asyncio
 from pathlib import Path
 
+# Force stdlib asyncio policy before any subprocess (stpyvista's trame
+# export runs in multiprocessing.Process and inherits the parent's
+# policy). uvicorn ships uvloop as a transitive of Streamlit; if the
+# parent's policy is uvloop, the 3-D Twin tab's subprocess hits
+# `nest_asyncio2._patch_loop` which raises `ValueError: Can't patch loop
+# of type <class 'uvloop.Loop'>`. The fix is to reset the policy at
+# import time — Streamlit's tornado server is unaffected.
+asyncio.set_event_loop_policy(asyncio.DefaultEventLoopPolicy())
+
 import numpy as np
 import streamlit as st
 
@@ -368,7 +378,23 @@ def _badge(ok: bool, label: str, detail: str):
             patch, edited_alignment, segments,
             z_exaggeration=float(z_exag), off_screen=True,
         )
-        stpyvista(plotter, key="ropeway_twin")
+        # Graceful fallback: stpyvista spawns a multiprocessing.Process for
+        # trame's HTML export. On systems where uvloop is loaded that
+        # subprocess can still fail (the asyncio policy reset at import
+        # time covers the common case but not e.g. pre-imported uvloop in
+        # the parent). Catch any subprocess crash, log it, and offer the
+        # alignment plot as a fallback so the rest of the tab stays usable.
+        try:
+            stpyvista(plotter, key="ropeway_twin")
+        except Exception as exc:  # noqa: BLE001 — surface any backend crash
+            st.warning(
+                "3-D Twin renderer crashed in its subprocess "
+                f"(`{type(exc).__name__}`). Falling back to the 2-D "
+                "alignment plot. Run `ropeway view3d --dem ... --out twin.html` "
+                "from a shell to get an interactive 3-D HTML you can open "
+                "directly in a browser."
+            )
+            st.exception(exc)
 
     # Persist edited layout into session for the Projects tab
     st.session_state["edited_alignment"] = edited_alignment