Phase 3: x86 (32-bit) game support via injector helper

Drive a nested Win32 sub-build (CMake ExternalProject, re-entrant via
COOP_X86_HELPER_BUILD) from the normal x64 build to produce coop_hook_x86.dll and
a 32-bit coop_inject_x86.exe, staged next to the x64 binaries. The host detects a
WOW64 target with IsWow64Process2 and spawns the helper to load the x86 DLL, since
a 64-bit process can't cleanly inject a 32-bit one. The shared-memory IPC is
fixed-width / bitness-stable, so the x64 host and x86 hook interoperate.

Validated end-to-end against Slaps and Beans (32-bit D3D11): all 15 hooks
installed, heartbeat advancing, the Present hook engaged (shared a 1920x1080
backbuffer -- the real-game video-hook proof Phantom Brave's D3D9 couldn't give),
and status/audio/video/log IPC all crossed the x64<->x86 boundary. coop_audio_probe
now also delegates to the helper for WOW64 targets. All 5 tests pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

host/src/inject/injector.hpp

@@ -11,12 +11,14 @@ enum class InjectStatus
 {
 	Ok,
 	OpenProcessFailed, // insufficient rights (try running the host as admin)
-	BitnessMismatch,   // 32-bit target; needs the x86 hook (later phase)
+	BitnessMismatch,   // 32-bit target; the x86 hook/helper aren't available
 	DllNotFound,
 	AllocFailed,
 	WriteFailed,
 	RemoteThreadFailed,
 	RemoteLoadFailed, // LoadLibraryW returned null inside the target
+	HelperNotFound,   // 32-bit target but coop_inject_x86.exe / coop_hook_x86.dll missing
+	HelperFailed,     // the x86 injector helper ran but reported failure
 };
 
 struct InjectResult
@@ -27,8 +29,9 @@ struct InjectResult
 
 const char* to_string(InjectStatus status);
 
-// Injects `dll_path` (absolute) into the process with `pid`. The host and DLL
-// must match the target's bitness; 32-bit targets are rejected up front.
+// Injects `dll_path` (absolute, the x64 coop_hook.dll) into `pid`. For a 32-bit
+// (WOW64) target the x64 host can't inject directly, so it spawns the sibling
+// coop_inject_x86.exe to load coop_hook_x86.dll (both expected next to dll_path).
 InjectResult inject_dll(unsigned long pid, const std::wstring& dll_path);
 
 } // namespace coop