Show error and diagnostics when st.plotly_chart fails instead of silent pass

Previously the st.plotly_chart call was wrapped in 'except Exception: pass'
which silently swallowed all rendering errors. The user would see no chart
and no error message.

Now:
- Exception message is shown via st.error()
- Diagnostics JSON is shown when debug is enabled (EXPLORER_DEBUG_TRAJECTORIES=1
  or UI checkbox), even when trace_count > 0

This reveals the actual root cause when the chart fails to render.

explorer.py

@@ -2096,8 +2096,18 @@ def choose_trajectory_title(axis_def: dict, axis: str, threshold: float = 0.65)
     else:
         try:
             st.plotly_chart(fig, use_container_width=True)
+        except Exception as e:
+            st.error(f"Trajectories rendering failed: {e}")
+            # Always show diagnostics when rendering fails, regardless of trace_count
+            if get_debug_trajectories_enabled():
+                try:
+                    st.json(_last_trajectories_diagnostics)
                 except Exception:
-            pass
+                    st.text_area(
+                        "Trajectories diagnostics (JSON failed)",
+                        json.dumps(_last_trajectories_diagnostics, default=str),
+                        height=240,
+                    )
 
 
 # ---------------------------------------------------------------------------