docs(expo): zfh engine and spm clarification

Author: Preeternal

README.md

@@ -523,8 +523,10 @@ example and platform-specific setup notes.
 }
 ```
 
-The Expo plugin writes the Android Gradle property, CocoaPods' `ZFH_ENGINE`, and
-the `ZFHEngine` value used by SwiftPM.
+The Expo plugin sets the Android Gradle property and CocoaPods' `ZFH_ENGINE`.
+It also mirrors the selected engine to `ZFHEngine` in `Info.plist`, but Expo
+Prebuild currently uses CocoaPods on iOS. This key does not enable SwiftPM; it
+is kept for forward compatibility with React Native's SwiftPM selector.
 
 If `engine` is omitted, `native` is used.
 

app.plugin.js

@@ -100,8 +100,8 @@ function updatePodfileContents(contents, engine) {
 
 function setIosEngine(config, engine) {
     config = withInfoPlist(config, (mod) => {
-        // SwiftPM's autolinking plugin compiles only the engine selected here.
-        // no key means native, matching the CocoaPods default.
+        // Expo Prebuild currently uses CocoaPods on iOS. Keep this value in
+        // sync for React Native's SwiftPM selector; it does not enable SwiftPM.
         mod.modResults.ZFHEngine = engine;
         return mod;
     });