feat(windows): make the Qt plugin backend cross-compilable

Three changes, all needed before any module can target mingw:

* meta.platforms gained windows in the three generated derivations. Without
  it every module fails at EVALUATION with "not available on the requested
  hostPlatform".

* getPluginFilename returned ".so" on Windows while getLibExtension two
  lines above already answered "dll". They disagreed because each carried
  its own copy of the platform branch, so the attribute set is now `rec`
  and getPluginFilename REUSES getLibExtension rather than repeating it.

* wrapQtAppsNoGuiHook is gated behind !isWindows and dontWrapQtApps is set
  on both plugin derivations. Both halves are required and neither is
  optional: the hook does not even EVALUATE for a mingw host, and would be
  inert anyway (wrap-qt-apps-hook skips anything that is not ELF or
  Mach-O), while qtbase's own setup hook hard-errors in qtPreHook unless
  dontWrapQtApps is set.

lib/buildPlugin.nix

@@ -198,6 +198,13 @@ in {
     libExt = gen.libExt;
 
   in pkgs.stdenv.mkDerivation (commonArgs // {
+    # Required wherever the Qt wrapper hooks are absent (Windows -- see
+    # common.nix): qtbase's setup hook hard-errors in qtPreHook with
+    # "depends on qtbase, but no wrapping behavior was specified" unless one of
+    # the two is present. Harmless elsewhere: these are plugins, not
+    # applications, so there is nothing to wrap.
+    dontWrapQtApps = true;
+
     pname = "${commonArgs.pname}-lib";
 
     inherit src;
@@ -366,6 +373,13 @@ in {
     };
 
   in pkgs.stdenv.mkDerivation (commonArgs // {
+    # Required wherever the Qt wrapper hooks are absent (Windows -- see
+    # common.nix): qtbase's setup hook hard-errors in qtPreHook with
+    # "depends on qtbase, but no wrapping behavior was specified" unless one of
+    # the two is present. Harmless elsewhere: these are plugins, not
+    # applications, so there is nothing to wrap.
+    dontWrapQtApps = true;
+
     pname = "${commonArgs.pname}-generated";
 
     inherit src;

lib/common.nix

@@ -5,9 +5,13 @@
 # and it's passed in by the caller.
 { lib }:
 
-{
-  # Supported target systems
-  systems = [ "aarch64-darwin" "x86_64-darwin" "aarch64-linux" "x86_64-linux" ];
+# `rec` so getPluginFilename can reuse getLibExtension instead of repeating the
+# platform branch -- the duplication is exactly what let them disagree about
+# Windows.
+rec {
+  # Supported target systems. "x86_64-windows" is a pseudo-system: a cross
+  # derivation's `system` attribute is its BUILD platform.
+  systems = [ "aarch64-darwin" "x86_64-darwin" "aarch64-linux" "x86_64-linux" "x86_64-windows" ];
 
   # Determine library extension based on platform
   getLibExtension = pkgs:
@@ -15,16 +19,26 @@
     else if pkgs.stdenv.hostPlatform.isWindows then "dll"
     else "so";
 
-  # Get the plugin filename for a module
+  # Get the plugin filename for a module.
+  #
+  # Reuses getLibExtension rather than repeating the branch: this used to have
+  # its own darwin/else test and returned ".so" on Windows, disagreeing with
+  # getLibExtension two lines above, which already answered "dll".
   getPluginFilename = pkgs: name:
-    "${name}_plugin.${if pkgs.stdenv.hostPlatform.isDarwin then "dylib" else "so"}";
+    "${name}_plugin.${getLibExtension pkgs}";
 
   # Qt-specific native build inputs
   commonNativeBuildInputs = pkgs: [
     pkgs.cmake
     pkgs.ninja
     pkgs.pkg-config
-    pkgs.qt6.wrapQtAppsNoGuiHook
+    # Mandatory guard, not an optimisation: wrapQtAppsNoGuiHook does not even
+    # EVALUATE for a mingw host, and would be inert anyway -- wrap-qt-apps-hook
+    # skips anything that is not ELF or Mach-O, so a PE is never wrapped.
+    # qtbase's own setup hook then hard-errors in qtPreHook unless
+    # dontWrapQtApps is set, which is why both halves are needed.
+  ] ++ pkgs.lib.optional (!pkgs.stdenv.hostPlatform.isWindows)
+    pkgs.qt6.wrapQtAppsNoGuiHook ++ [
   ];
 
   # Qt-specific build inputs

lib/default.nix

@@ -51,7 +51,7 @@ in {
       } // extraEnv;
       meta = with lib; {
         description = config.description;
-        platforms = platforms.unix;
+        platforms = platforms.unix ++ platforms.windows;
       };
     };
   in mkBuildPlugin.build {
@@ -93,7 +93,7 @@ in {
       } // extraEnv;
       meta = with lib; {
         description = config.description;
-        platforms = platforms.unix;
+        platforms = platforms.unix ++ platforms.windows;
       };
     };
   in mkBuildPlugin.generate {
@@ -122,7 +122,7 @@ in {
       version = config.version;
       meta = with lib; {
         description = config.description;
-        platforms = platforms.unix;
+        platforms = platforms.unix ++ platforms.windows;
       };
     };
   in mkBuildHeaders.build {