macos: fix extern C SubprocessMacInit declaration scope

extern "C" linkage specification must be at namespace (file) scope in
C++, not inside a function body.  Move the SubprocessMacInit() forward
declaration out of main() into file scope, guarded by OS_MAC.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: linesight

src/subprocess/main.cpp

@@ -28,18 +28,21 @@ int APIENTRY wWinMain(HINSTANCE hInstance,
 
 #include <string.h>
 
+#if defined(OS_MAC)
+// CEF 130+: MachPortRendezvousClientMac builds the service name as
+//   CFBundleIdentifier + ".MachPortRendezvousServer." + parent_pid
+// The browser process injects CFBundleIdentifier = "org.cefpython" in
+// MacInitialize() before CefInitialize().  This subprocess binary is a flat
+// binary (not an app bundle), so CFBundleGetMainBundle() returns no
+// CFBundleIdentifier, producing a lookup name starting with "." that never
+// matches the registered service.  Call SubprocessMacInit() to inject the
+// same identifier so bootstrap_look_up finds the server.
+extern "C" void SubprocessMacInit();
+#endif
+
 int main(int argc, char **argv)
 {
 #if defined(OS_MAC)
-	// CEF 130+: MachPortRendezvousClientMac builds the service name as
-	//   CFBundleIdentifier + ".MachPortRendezvousServer." + parent_pid
-	// The browser process injects CFBundleIdentifier = "org.cefpython" in
-	// MacInitialize() before CefInitialize().  This subprocess binary is a flat
-	// binary (not an app bundle), so CFBundleGetMainBundle() returns no
-	// CFBundleIdentifier, producing a lookup name starting with "." that never
-	// matches the registered service.  Call SubprocessMacInit() to inject the
-	// same identifier so bootstrap_look_up finds the server.
-	extern "C" void SubprocessMacInit();
 	SubprocessMacInit();
 #endif
 #if defined(OS_LINUX)