Document the win64/WoW64 Wine prefix needed to cross-build the .exe

electron-winstaller's Squirrel runs as 32-bit .NET but zips the package by
ShellExecuteEx-ing the 64-bit vendor/7z.exe. A 32-bit-only Wine prefix
('#arch=win32') cannot launch a 64-bit PE, so the cross-build fails with
'ShellExecuteEx failed: Bad EXE format'. Record the WINEARCH=win64 recipe
(and the pre-existing Wine+Mono requirement) instead of leaving it as a
build-host surprise.

packaging/wsl2/README.md

@@ -97,6 +97,22 @@ The installer is written to `out/make/squirrel.windows/x64/` as
   `You must install both Mono and Wine on non-Windows`. Install e.g.
   `wine` and `mono-complete` (or `mono-core`) first.
 
+  It also needs a **64-bit (WoW64) Wine prefix**. Squirrel is a 32-bit .NET
+  assembly but zips the package by `ShellExecuteEx`-ing the 64-bit
+  `node_modules/electron-winstaller/vendor/7z.exe`; a 32-bit-only prefix
+  (`#arch=win32`) cannot launch a 64-bit PE and the build fails with
+  `ShellExecuteEx failed: Bad EXE format`. Build against a WoW64 prefix without
+  disturbing an existing one:
+
+  ```sh
+  export WINEARCH=win64 WINEPREFIX="$HOME/.wine-trimsp64"
+  wineboot -u
+  npm run make:win
+  ```
+
+  (Wine ≥ 9 supports running 32-bit apps from a `win64` prefix, so a single
+  prefix is enough.) On Windows none of this applies.
+
 ### Verifying the shipped .exe
 
 The `TrimSP.exe` inside the install loads the engine only through WSL, so the
@@ -144,5 +160,6 @@ workbench is often driven remotely. After the first-run import, verify:
 - `wsl.exe` availability is assumed; older Windows builds without WSL2 must
   install it (`wsl --install`) first.
 - The macOS runtime is not built.
-- Cross-building the `.exe` from Linux additionally needs Wine and Mono (see
-  above); building on Windows needs neither.
+- Cross-building the `.exe` from Linux additionally needs Wine, Mono and a
+  64-bit (WoW64) Wine prefix (see above); building on Windows needs none of
+  these.