fix(ci): use xcodebuild archive to produce .app for IPA packaging

xcodebuild build on an SPM executableTarget only outputs the module/bundle,
not a .app. archive produces a .xcarchive with the .app under Products/.

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

codemagic.yaml

@@ -22,11 +22,11 @@ workflows:
     scripts:
       - name: Build (unsigned)
         script: |
-          xcodebuild build \
+          xcodebuild archive \
             -scheme SilverApple \
             -destination "generic/platform=iOS" \
             -configuration Release \
-            -derivedDataPath "$CM_BUILD_DIR/DerivedData" \
+            -archivePath "$CM_BUILD_DIR/SilverApple.xcarchive" \
             CODE_SIGNING_ALLOWED=NO \
             CODE_SIGNING_REQUIRED=NO \
             CODE_SIGN_IDENTITY="" \
@@ -35,10 +35,10 @@ workflows:
       - name: Package IPA
         script: |
           cd "$CM_BUILD_DIR"
-          APP=$(find DerivedData/Build/Products -name "*.app" -type d | head -1)
+          APP=$(find SilverApple.xcarchive/Products -name "*.app" -type d | head -1)
           if [ -z "$APP" ]; then
-            echo "ERROR: No .app found. Products:"
-            find DerivedData/Build/Products -type d | head -30
+            echo "ERROR: No .app found in xcarchive. Contents:"
+            find SilverApple.xcarchive -type d | head -30
             exit 1
           fi
           echo "Packaging: $APP"