CI: run the suite on every platform the README now advertises

Gitea Actions matrix: ubuntu-latest (py3.11/3.13) plus Debian 12,
Ubuntu 24.04, Fedora and Arch containers, each installing the system
libraries PySide6 wheels dlopen - all four validated locally in
Docker (Debian 141/141, QtWidgets import probe green everywhere).
Windows/macOS and a NixOS-FHS job (tools/ci-fhs.nix, the library set
from docs/NIXOS.md) are gated to github.com runners so they skip
instead of queueing forever here. README gets a Platform support
table with the workflow badge; KiCad-in-the-loop stays field-tested,
CI covers the solver and fallback suite.

tools/ci-fhs.nix

@@ -0,0 +1,25 @@
+# CI-only: the FHS environment from docs/NIXOS.md minus KiCad itself.
+# The suite runs against pip wheels exactly as the plugin's venv does
+# inside the wrapped KiCad: PySide6 dlopens these libraries at FHS
+# paths. Keep this library list in sync with the buildFHSEnv recipe in
+# docs/NIXOS.md — CI failing here means the documented recipe broke.
+{ pkgs ? import <nixpkgs> { } }:
+
+pkgs.buildFHSEnv {
+  name = "fill-resistance-ci";
+  targetPkgs = p: with p; [
+    bashInteractive curl cacert
+    glib fontconfig freetype dbus libGL libxkbcommon
+    xcb-util-cursor wayland zlib zstd.out
+    xorg.libX11 xorg.libxcb xorg.libXext xorg.libXrender
+    xorg.libSM xorg.libICE xorg.libXrandr xorg.libXi
+    xorg.libXcursor xorg.libXfixes
+    # xcb-util family needed by PySide6's bundled xcb platform plugin
+    xorg.xcbutil xorg.xcbutilwm xorg.xcbutilimage
+    xorg.xcbutilkeysyms xorg.xcbutilrenderutil
+  ];
+  # See docs/NIXOS.md failure layer 3: the desktop's QT_PLUGIN_PATH
+  # must not leak into the wheel's bundled Qt.
+  profile = "unset QT_PLUGIN_PATH";
+  runScript = "bash";
+}