feat: add IHasDspPresetSave interface for DSP preset management

src/PepperDash.Essentials.Core/Devices/IHasDspPresetSave.cs

@@ -0,0 +1,14 @@
+namespace PepperDash.Essentials.Core
+{
+    /// <summary>
+    /// Defines the contract for IHasDspPresetSave
+    /// </summary>
+    public interface IHasDspPresetSave : IDspPresets // recall + save
+    {
+        /// <summary>
+        /// Saves the DSP preset by key
+        /// </summary>
+        /// <param name="presetKey"></param>
+        void SavePresetByKey(string presetKey); // mirrors RecallPreset(string key)
+    }
+}