feat: add GatedInserter, SwappableTranscriber and LevelSource

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

Sources/SstCore/AppGlue.swift

@@ -0,0 +1,68 @@
+import Foundation
+
+/// Inserts text only while `shouldInsert()` says so (not when Sst itself is in front, nor for window recordings).
+public struct GatedInserter: TextInserting {
+    private let inner: TextInserting
+    private let shouldInsert: () -> Bool
+
+    public init(inner: TextInserting, shouldInsert: @escaping () -> Bool) {
+        self.inner = inner
+        self.shouldInsert = shouldInsert
+    }
+
+    public func insert(_ text: String) async throws {
+        guard shouldInsert() else { return }
+        try await inner.insert(text)
+    }
+}
+
+/// Lets the app replace the speech model (for example after a model switch) without rebuilding the controller.
+public final class SwappableTranscriber: Transcriber, @unchecked Sendable {
+    public enum SwapError: Error, LocalizedError {
+        case noTranscriber
+
+        public var errorDescription: String? { "Spraakmodel is nog niet geladen." }
+    }
+
+    private let lock = NSLock()
+    private var current: Transcriber?
+
+    public init(_ initial: Transcriber? = nil) {
+        current = initial
+    }
+
+    public func swap(to transcriber: Transcriber?) {
+        lock.withLock { current = transcriber }
+    }
+
+    public var hasTranscriber: Bool {
+        lock.withLock { current != nil }
+    }
+
+    public func transcribe(_ samples: [Float]) async throws -> String {
+        try await transcribe(samples, vocabulary: [])
+    }
+
+    public func transcribe(_ samples: [Float], vocabulary: [String]) async throws -> String {
+        guard let transcriber = lock.withLock({ current }) else { throw SwapError.noTranscriber }
+        return try await transcriber.transcribe(samples, vocabulary: vocabulary)
+    }
+}
+
+/// The latest input level, written from the audio thread and read by the meter once per frame.
+public final class LevelSource: @unchecked Sendable {
+    private let lock = NSLock()
+    private var value = AudioLevel.silenceFloorDbfs
+
+    public init() {}
+
+    public var dbfs: Float { lock.withLock { value } }
+
+    public func update(_ dbfs: Float) {
+        lock.withLock { value = dbfs }
+    }
+
+    public func reset() {
+        update(AudioLevel.silenceFloorDbfs)
+    }
+}