try running on platforms too

.github/workflows/check.yaml

@@ -18,3 +18,12 @@ jobs:
       # https://forums.swift.org/t/warnings-as-errors-for-libraries-frameworks/58393/2
       - run: swift build -Xswiftc -warnings-as-errors
       - run: swift test
+
+  check-on-platforms:
+    strategy:
+      matrix:
+        platform: [macOS, iOS, tvOS]
+    steps:
+      # TODO OS version, and ambiguity in platforms, and no match
+      - run: xcodebuild -scheme AblyChat -platform ${{ matrix.platform }}
+      - run: xcodebuild test -scheme AblyChat -platform ${{ matrix.platform }}

CONTRIBUTING.md

@@ -29,3 +29,5 @@ At time of writing, the latest version of Xcode is 15.4, and the `Package.swift`
 and then we also take the minimum of these and ably-cocoa (which at time of writing were all lower than the above)
 
 TODO let's see what Xcode 16 says too though — OK, doesn’t seem any different in Beta 3
+
+TODO we probably also have to take into account which simulators are available on the version of Xcode we're using, _and_ which are installed on the GitHub runner