Run native Windows DLSS K and M through an explicit OptiScaler source backend

patches/optiscaler/README.md

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+# Explicit Windows d4r backend
+
+Base: OptiScaler `45a2001303ddff632e279f77aef85ceede5832cb` (2026-09-30).
+The modifications to OptiScaler are GPL-3.0, matching its upstream license.
+No NVIDIA DLL is supplied or patched.
+
+`0001` introduces `[DLSS] AllowExternalBackend=true`, default false. Set
+`[Libraries] NvngxPath` to the absolute directory containing the Windows d4r
+shim as `_nvngx.dll`. Discovery requires its GPU-free ABI-1 marker. Physical
+adapter support is queried through the bridge's D3D12 requirements API after
+DLL startup; vendor, LUID and HIP architecture are retained.
+
+The configured `D4R_NGX_CORE` absolute path bypasses game-facing NGX DLL
+redirection. The external backend loads its own NVAPI provider. An explicitly
+selected external DLSS backend reports initialization/load errors instead of
+silently switching to FSR. The patch fixes the upstream loader's filesystem
+exception by making IsSubpath a lexical check, and retains a PDB for diagnosis.
+
+Build with Microsoft Build Tools 2022, v143 and Windows SDK 10.0.26100:
+
+```powershell
+.\scripts\windows\build-optiscaler-windows.ps1
+```
+
+The script clones the pinned source if absent, checks/applies the patch,
+initializes pinned submodules, invokes MSBuild and writes the DLL, PDB, license
+and build metadata into `dist/optiscaler-windows-d4r`. Existing source is not
+reset. The upstream packaging post-build event is disabled for this build.
+The official bootstrapper is https://aka.ms/vs/17/release/vs_buildtools.exe;
+install the `Microsoft.VisualStudio.Workload.VCTools`,
+`Microsoft.VisualStudio.Component.VC.Tools.x86.x64` and
+`Microsoft.VisualStudio.Component.Windows11SDK.26100` components. The audited
+local build uses MSVC 14.44.35207 / Build Tools 17.14.
+
+For standalone verification add
+`-OptiScalerDll <absolute-dist-path>/OptiScaler.dll -CaptureExceptions`
+to the documented K/M runner. Its private INI selects DLSS, disables frame
+generation and keeps RestoreComputeSignature, RestoreGraphicSignature and
+ExtendedStateRestore false. Use `-RequireNativeNetwork`; finite output alone
+cannot distinguish DLSS from an unwanted FSR replacement.