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/Playback/MixPresets.swift

@@ -0,0 +1,50 @@
+import Foundation
+
+/// A one-tap stem mix. [build] returns nil when the track lacks the stems it needs.
+struct MixPreset: Identifiable, Equatable {
+    let label: String
+    let build: ([String]) -> [String: StemSetting]?
+    var id: String { label }
+    static func == (a: MixPreset, b: MixPreset) -> Bool { a.label == b.label }
+}
+
+enum MixPresets {
+    static let full = MixPreset(label: "Full Mix") { _ in [:] }
+    static let karaoke = MixPreset(label: "Karaoke") { n in n.contains("vocals") ? ["vocals": StemSetting(muted: true)] : nil }
+
+    static let all: [MixPreset] = [
+        full,
+        karaoke,
+        MixPreset(label: "A Cappella") { n in n.contains("vocals") ? ["vocals": StemSetting(solo: true)] : nil },
+        MixPreset(label: "No Drums") { n in n.contains("drums") ? ["drums": StemSetting(muted: true)] : nil },
+        MixPreset(label: "Rhythm") { n in
+            n.contains("drums") && n.contains("bass") ? ["drums": StemSetting(solo: true), "bass": StemSetting(solo: true)] : nil
+        },
+        MixPreset(label: "No Guitar") { n in n.contains("guitar") ? ["guitar": StemSetting(muted: true)] : nil },
+        MixPreset(label: "No Bass") { n in n.contains("bass") ? ["bass": StemSetting(muted: true)] : nil },
+    ]
+
+    /// Drops stems left at their defaults, so mixes compare by what they actually change.
+    static func normalized(_ m: [String: StemSetting]) -> [String: StemSetting] { m.filter { $0.value != StemSetting() } }
+
+    /// The preset [mix] matches for these stems, or nil for a hand-made mix.
+    static func current(_ names: [String], _ mix: [String: StemSetting]) -> MixPreset? {
+        let m = normalized(mix)
+        return all.first { p in p.build(names).map { normalized($0) == m } ?? false }
+    }
+
+    /// The preset after the current one (a custom mix goes back to Full), skipping ones these stems can't do.
+    static func next(_ names: [String], _ mix: [String: StemSetting]) -> MixPreset {
+        let usable = all.filter { $0.build(names) != nil }
+        guard let cur = current(names, mix), let i = usable.firstIndex(of: cur) else { return full }
+        return usable[(i + 1) % usable.count]
+    }
+
+    /// True when the vocals stem can't be heard (muted, or another stem soloed).
+    static func vocalsSilent(_ names: [String], _ mix: [String: StemSetting]) -> Bool {
+        if !names.contains("vocals") { return false }
+        let v = mix["vocals"] ?? StemSetting()
+        let anySolo = names.contains { mix[$0]?.solo == true }
+        return v.muted || anySolo && !v.solo || v.level <= 0
+    }
+}