feat: migrate on-device Qwen3 ASR to CoreML for background Flow dictation

Replace MLX GPU inference with CoreML bundles so transcription continues
while the host app is backgrounded. Adds model download and warm-up,
vendored Qwen3Speech, and updates onboarding, settings, and copy for the
~1.6 GB CoreML package (iOS 18+).

OSGKeyboardShared/Models/OnDeviceModel.swift

@@ -0,0 +1,54 @@
+// OnDeviceModel.swift
+// OSGKeyboard · Shared
+//
+// Identity of an on-device model the host app downloads and the
+// keyboard extension observes via App Group flags (the extension
+// cannot read the main app's Caches directory).
+
+import Foundation
+
+public enum OnDeviceModel: String, CaseIterable, Identifiable, Sendable {
+    case qwen3ASR
+
+    public var id: String { rawValue }
+
+    /// CoreML inference bundle (`aufklarer/Qwen3-ASR-CoreML`), derived from
+    /// official `Qwen/Qwen3-ASR-0.6B`.
+    public var repoId: String {
+        switch self {
+        case .qwen3ASR: return "aufklarer/Qwen3-ASR-CoreML"
+        }
+    }
+
+    /// Tokenizer files (vocab / merges) pulled from the upstream Qwen repo.
+    public var tokenizerRepoId: String {
+        switch self {
+        case .qwen3ASR: return "Qwen/Qwen3-ASR-0.6B"
+        }
+    }
+
+    public var displayName: String {
+        switch self {
+        case .qwen3ASR: return "Qwen3-ASR 0.6B (CoreML)"
+        }
+    }
+
+    public var approximateSizeMB: Int {
+        switch self {
+        case .qwen3ASR: return 1_600
+        }
+    }
+
+    public var compactSizeLabel: String {
+        "\(approximateSizeMB)M"
+    }
+
+    /// Settings list title: model name plus compact size.
+    public var listTitle: String {
+        "\(displayName) · \(compactSizeLabel)"
+    }
+
+    public var repoAndSizeLabel: String {
+        "\(repoId) · \(approximateSizeMB) MB"
+    }
+}