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>

README.md

@@ -92,6 +92,14 @@ OS specifics are spelled out per step and in *Platform notes* below.
   On **ARM64 (aarch64)** there are no pyamg wheels —
   `requirements.txt` skips pyamg there and the solver falls back to
   Jacobi-CG: same results, noticeably slower on large grids.
+  **NixOS**: pip's Linux wheels link against standard FHS library
+  paths, which NixOS does not provide — PySide6 fails with
+  `libgthread-2.0.so.0: cannot open shared object file`. The plugin
+  cannot fix this from inside its venv (KiCad installs wheels only):
+  run KiCad inside an FHS environment (e.g. `steam-run kicad`) or
+  enable `programs.nix-ld` with Qt's runtime libraries (glib,
+  fontconfig, freetype, dbus, libGL, libxkbcommon and the X11/xcb
+  set).
 
 ## Usage