build: pin the project to Win32 with a preset and a configure guard

WoW.exe is x86, so there is no valid 64-bit configure. Nothing in the
repo said so, which let VS Code decide instead: CMake Tools configures on
folder open by default, and with no presets file it takes the
architecture from a scanned kit. Here it chose the x86_amd64 cross kit
(-T host=x86 -A x64) and flipped the shared build directory to x64.

That failure is badly disguised. A 64-bit build first shows up as a
static_assert deep in a struct-layout check, which reads as an unrelated
code fault rather than a build-configuration one.

Add CMakePresets.json with a single win32 configure preset. A presets file
makes CMake Tools use presets and stop using kits, so the architecture
becomes a property of the project instead of whichever kit a machine
happens to select. The preset names no generator, matching the documented
command line: CMake then resolves the newest installed Visual Studio,
which a pinned version would get wrong. Schema version 3 is the earliest
that allows omitting the generator.

Add a CMAKE_SIZEOF_VOID_P guard after project() for whatever gets past
the preset -- a hand-typed -A x64, or a stale cache -- so the configure
fails with the recovery steps instead of building something unloadable.

The existing `cmake -B build -A Win32` flow keeps working; presets are
opt-in on the command line.

CMakeLists.txt

@@ -2,6 +2,21 @@ cmake_minimum_required(VERSION 3.10)
 
 project(ClassicAPI LANGUAGES CXX RC)
 
+# WoW.exe is x86, so the injected DLL must be x86 too — there is no valid
+# 64-bit configure of this project. Fail here rather than let one through:
+# a 64-bit build's first visible symptom is a static_assert deep in a
+# struct-layout check, which reads as an unrelated code fault. An IDE that
+# configures on its own (VS Code's CMake Tools picks an architecture from a
+# scanned kit) can flip an existing build directory this way, which is what
+# CMakePresets.json's `win32` preset exists to prevent.
+if(NOT CMAKE_SIZEOF_VOID_P EQUAL 4)
+    message(FATAL_ERROR
+        "ClassicAPI builds 32-bit only. This configure uses "
+        "${CMAKE_SIZEOF_VOID_P}-byte pointers. Delete the build directory, "
+        "because CMake cannot switch platform in place. Then configure with "
+        "`cmake --preset win32` or `cmake -B build -A Win32`.")
+endif()
+
 set(CMAKE_CXX_STANDARD 17)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 set(CMAKE_MSVC_RUNTIME_LIBRARY "MultiThreaded")

CMakePresets.json

@@ -0,0 +1,25 @@
+{
+  "version": 3,
+  "cmakeMinimumRequired": {
+    "major": 3,
+    "minor": 21,
+    "patch": 0
+  },
+  "configurePresets": [
+    {
+      "name": "win32",
+      "displayName": "Win32 (x86)",
+      "description": "The only supported configuration: WoW.exe is 32-bit, so the DLL must be 32-bit too. No generator is named, so CMake picks the newest installed Visual Studio, matching the documented command line.",
+      "binaryDir": "${sourceDir}/build",
+      "architecture": "Win32"
+    }
+  ],
+  "buildPresets": [
+    {
+      "name": "release",
+      "displayName": "Release",
+      "configurePreset": "win32",
+      "configuration": "Release"
+    }
+  ]
+}