ASI add-on for existing OptiScaler installs

Add an optional D4R_INSTALL_MODE=asi release (d4r-<version>-asi.zip)
holding only d4r.asi and a companion d4r/ folder, to drop into an
existing OptiScaler plugin directory.

- Shim: export InitializeASI. It identifies OptiScaler by its PE export
  name to find the active INI, then launches d4r-setup.exe out of
  process (no GPU or network work under the loader lock). The .asi
  build loads its runtime and config from the companion d4r/ folder.

- tools/d4r_asi_setup.cpp: native helper. It waits for the game to
  exit, installs the D3D12 wrapper and the private d4r12core.dll beside
  the EXE, and merges the NGX route into the loader's INI (including
  GE-Proton's prefix-local INI) with backups. Unchanged launches do
  nothing. It skips Proton helpers and background utilities and runs
  setup for one plugin install at a time.

- scripts/private_d3d12_core.py: points vkd3d's d3d12.dll forwarder at
  d4r12core.dll without moving code addresses.

- install.sh runs from inside the ASI's d4r/ as well as the proxy
  release root. d4r-check.sh checks ASI installs (loader, plugin path,
  LoadAsiPlugins, HookOriginalNvngxOnly, Wine drive paths).

- Packaging, build script, docs, ASI_README.txt and tests.

packaging/D4R_README.txt

@@ -39,6 +39,10 @@ What you need
 
 Install
 -------
+For an existing game-local OptiScaler installation, the separate -asi.zip add-on
+uses its ASI loader and keeps its proxy DLL and unrelated INI settings. Follow
+that archive's d4r/README.txt: setup runs automatically when the ASI loads.
+
 1. Extract everything in this zip into the folder that holds the game's main .exe. For Unreal
    Engine games that is <game>/<Project>/Binaries/Win64/, next to <Project>-Win64-Shipping.exe.
    You get dxgi.dll, OptiScaler.ini, d3d12.dll, d3d12core.dll, this file and a d4r folder.