Fail usefully on NixOS: probe QtCore, explain the missing-.so error

First NixOS attempt: pip wheel PySide6 cannot load libgthread-2.0.so.0
because NixOS has no FHS library paths. Nothing inside the venv can
fix that (KiCad installs wheels only) - but the plugin made it worse
twice over:

- The backend probe imported bare PySide6, whose pure-Python __init__
  succeeds even when QtCore's .so cannot load - so matplotlib was
  promised QtAgg and the error figure died at switch_backend, taking
  the failure report with it. The probe now imports <binding>.QtCore
  and falls through to Tk/Agg on a broken Qt.
- The user got a raw ImportError traceback. A cannot-open-shared-object
  failure during the kipy/dialog imports now raises a UserFacingError
  that names the actual fixes: run KiCad in an FHS environment
  (steam-run) or enable nix-ld with Qt runtime libraries. The README
  Linux notes carry the same guidance.

141 passed on the dev stack and the Python 3.9 mac-equivalent stack.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

fill_resistance/plots.py

@@ -24,10 +24,16 @@ def _pick_backend():
     dependency and the selection dialog / progress window put a Qt event
     loop in this process, after which matplotlib refuses TkAgg
     ("Cannot load backend 'TkAgg' ... as 'qt' is currently running") -
-    exactly what happened on macOS, whose bundled Python ships tkinter."""
+    exactly what happened on macOS, whose bundled Python ships tkinter.
+
+    The probe must import the binding's native core, not just the
+    package: on NixOS `import PySide6` succeeds (pure __init__) while
+    QtCore's .so cannot find the system libraries pip wheels expect
+    ("libgthread-2.0.so.0: cannot open shared object file") - promising
+    QtAgg then kills even the error figure at switch_backend time."""
     for qt in ("PySide6", "PyQt6", "PyQt5", "PySide2"):
         try:
-            __import__(qt)
+            __import__(qt + ".QtCore")
             return "QtAgg" if qt in ("PySide6", "PyQt6") else "Qt5Agg"
         except Exception:
             continue