optiscaler: package v0.9.1

stdenvNoCC + p7zip extraction; strips installer scripts and README,
keeps Licenses/. dontFixup since the artifacts are Windows DLLs.
meta.license is unfreeRedistributable to reflect the bundled XeSS
(Intel SLA) alongside the GPL-3.0 source.

Wires lib/overlays.nix into mkDesktopHost (was muffin-only) and adds
"optiscaler" to the unfree allowlist on jovian hosts so yarn can
consume it without flipping the global allowUnfree flag.

lib/overlays.nix

@@ -90,4 +90,67 @@ final: prev: {
     doCheck = false;
     meta.mainProgram = "mc-monitor";
   };
+
+  # OptiScaler: universal upscaler/frame-gen middleware. Bridges DLSS, XeSS,
+  # and FSR backends so games shipping older upscalers can be retargeted at,
+  # e.g., FSR 4 (RDNA 4 native, RDNA 3 INT8 via Mesa+Proton).
+  #
+  # Bundled binaries are unaudited Windows DLLs intended to be loaded under
+  # Wine/Proton. The release tarball ships its own license tree which we
+  # carry through verbatim under $out/Licenses for compliance.
+  #
+  # Bumping: drop the new asset URL + recompute hash. Verify $out still
+  # contains OptiScaler.dll and OptiScaler.ini (consumers reference both by
+  # name).
+  optiscaler = prev.stdenvNoCC.mkDerivation rec {
+    pname = "optiscaler";
+    version = "0.9.1";
+
+    src = prev.fetchurl {
+      url = "https://github.com/optiscaler/OptiScaler/releases/download/v${version}/Optiscaler_${version}-final.20260427._DSB.7z";
+      hash = "sha256-VtGhjjoy2XjAE0hE6AO6jPBBNCJs/NuCg/aNGAg2+rA=";
+    };
+
+    nativeBuildInputs = [ prev.p7zip ];
+
+    unpackPhase = ''
+      runHook preUnpack
+      mkdir -p source
+      7z x -bd -o"source" $src >/dev/null
+      runHook postUnpack
+    '';
+
+    sourceRoot = "source";
+
+    dontConfigure = true;
+    dontBuild = true;
+    # Windows DLLs; nothing to patchelf or strip.
+    dontFixup = true;
+
+    installPhase = ''
+      runHook preInstall
+      mkdir -p $out
+      cp -r ./* $out/
+      # The installer scripts and "extract me" README aren't useful at runtime.
+      rm -f $out/setup_linux.sh $out/setup_windows.bat
+      rm -f "$out/!! README_EXTRACT ALL FILES TO GAME FOLDER !!.txt"
+      runHook postInstall
+    '';
+
+    meta = with prev.lib; {
+      description = "Upscaler/frame-gen middleware bridging DLSS, XeSS, and FSR backends";
+      homepage = "https://github.com/optiscaler/OptiScaler";
+      # OptiScaler proper is GPL-3.0-or-later. The bundled FidelityFX SDK is
+      # MIT-0; XeSS ships its own Intel SLA. License texts are preserved at
+      # $out/Licenses/.
+      license = with licenses; [
+        gpl3Plus
+        mit0
+        unfreeRedistributable
+      ];
+      sourceProvenance = [ sourceTypes.binaryNativeCode ];
+      platforms = platforms.linux;
+      maintainers = [ ];
+    };
+  };
 }