Add iOS app with CarPlay and Siri

SwiftUI port of the Android app under ios/, with CarPlay browse and
Now Playing, Siri play-media intents, and a build-ipa.sh script that
makes an unsigned IPA for sideloading. README gets an iOS section and
the vibecoded disclaimer.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

ios/Stemify/App/SiriIntents.swift

@@ -0,0 +1,33 @@
+import Intents
+
+/// "Hey Siri, play … on Stemify". Handled in the app process, so it can start playback directly.
+final class PlayMediaHandler: NSObject, INPlayMediaIntentHandling {
+    func resolveMediaItems(for intent: INPlayMediaIntent) async -> [INPlayMediaMediaItemResolutionResult] {
+        let q = intent.mediaSearch?.mediaName ?? intent.mediaSearch?.artistName ?? intent.mediaSearch?.albumName ?? ""
+        let item = INMediaItem(identifier: "search:\(q)", title: q.isEmpty ? "Downloads" : q, type: .music, artwork: nil)
+        return INPlayMediaMediaItemResolutionResult.successes(with: [item])
+    }
+
+    func handle(intent: INPlayMediaIntent) async -> INPlayMediaIntentResponse {
+        let s = intent.mediaSearch
+        let q = s?.mediaName ?? s?.artistName ?? s?.albumName ?? intent.mediaItems?.first?.title ?? ""
+        let shuffle = intent.playShuffled == true
+        let ok = await Self.play(q, shuffle)
+        return INPlayMediaIntentResponse(code: ok ? .success : .failure, userActivity: nil)
+    }
+
+    @MainActor static func play(_ query: String, _ shuffle: Bool) async -> Bool {
+        let g = AppGraph.shared
+        do {
+            guard let r = try await g.car.resolveSearch(query) else {
+                g.playback.report(query.isEmpty ? "Nothing downloaded yet" : "Nothing found for “\(query)”")
+                return false
+            }
+            g.playback.playTracks(r.tracks, start: r.start, shuffle: r.shuffle || shuffle)
+            return true
+        } catch {
+            g.playback.report(error is NotSignedIn ? "Sign in to Plex on your phone" : "Couldn't load that: \(error.localizedDescription)")
+            return false
+        }
+    }
+}