NixOS works: document the full FHS recipe, drop the nix-ld dead end

First successful Linux field test (NixOS 26.05, Plasma 6, KiCad 10).
Getting there peeled three layers the error messages misattribute:
the FHS env itself (libgthread), then libs the xcb-cursor hint hides
(the xcb-util family, and libzstd for libQt6Core - in zstd.out, not
zstd's default output), and finally Plasma's QT_PLUGIN_PATH feeding
the system Qt's platform plugins to the wheel's bundled Qt, a
private-ABI mismatch that fails exactly like a missing library.

New docs/NIXOS.md carries the verified buildFHSEnv wrapper, the
steam-run quick test (now with env -u QT_PLUGIN_PATH), and a guide
for chasing future missing sonames. README upgrades Linux from
untested to works and links the doc. The in-plugin error no longer
suggests programs.nix-ld: the venv python is a Nix binary, so the
nix-ld shim never engages for it.

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

fill_resistance/main.py

@@ -53,9 +53,9 @@ def main() -> None:
                 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."
+                f"On NixOS, run KiCad inside an FHS environment "
+                f"(buildFHSEnv wrapper, or steam-run for a quick "
+                f"test) - see docs/NIXOS.md in the plugin repo."
             )
         try:
             kicad, board = board_io.connect()