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/main.py

@@ -39,9 +39,24 @@ def _fail(message: str, outdir) -> None:
 def main() -> None:
     outdir = None
     try:
-        from kipy.errors import ApiError
+        try:
+            from kipy.errors import ApiError
 
-        from . import board_io, dialog
+            from . import board_io, dialog
+        except ImportError as e:
+            if "cannot open shared object file" not in str(e):
+                raise
+            # pip's Linux wheels link against FHS system libraries;
+            # on NixOS those paths don't exist and PySide6/pynng die
+            # exactly like this. Nothing inside the venv can fix it.
+            raise UserFacingError(
+                f"A compiled dependency cannot load its system "
+                f"libraries: {e}\nThe plugin venv is built from pip "
+                f"wheels, which expect standard (FHS) library paths. "
+                f"On NixOS, run KiCad inside an FHS environment (e.g. "
+                f"steam-run) or enable programs.nix-ld with Qt's "
+                f"runtime libraries - see README, Platform notes."
+            )
         try:
             kicad, board = board_io.connect()
             stackup = board_io.get_stackup_info(board)