v0.1: Robovoice — PTT re-voicing app

WinForms tray app that captures text (from file or direct input) and
synthesizes speech via libpiper (Piper TTS), outputting to any audio
device (VB-CABLE for virtual mic routing).

Features:
- Global PTT hotkey (configurable F1-F12) via WH_KEYBOARD_LL
- Text file sequential reader (line-by-line on each PTT cycle)
- Direct text input with Speak button
- Voice manager: browse 147-voice Piper catalogue, download, remove
- libpiper P/Invoke wrapper with UTF-8 marshaling, streaming chunks
- BufferedWaveProvider streaming playback with trailing silence flush
- Sentence terminator auto-append (fixes espeak-ng final-word drop)
- Tray icon with minimize-to-tray

Architecture:
- Robovoice.Core: ITtsEngine, ISttSource interfaces, voice catalogue
- Robovoice.Tts.LibPiper: P/Invoke wrapper for libpiper.dll
- Robovoice.Stt.File: text file STT source (testing without mic server)
- Robovoice.Stt.Udp: UDP client stub (for future Linux mic server)
- Robovoice.App: WinForms UI, orchestrator, PTT hotkey, audio output

Robovoice.Tts.LibPiper/NativeDependencyLoader.cs

@@ -0,0 +1,28 @@
+using System.Runtime.InteropServices;
+
+namespace Robovoice.Tts.LibPiper;
+
+internal static class NativeDependencyLoader
+{
+    private static int _loaded;
+
+    public static void EnsureLoaded()
+    {
+        if (Interlocked.CompareExchange(ref _loaded, 1, 0) != 0)
+            return;
+
+        string baseDir = AppContext.BaseDirectory;
+        string nativeDir = Path.Combine(baseDir, "runtimes", "win-x64", "native");
+
+        if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows) && Directory.Exists(nativeDir))
+        {
+            if (!SetDllDirectory(nativeDir))
+                throw new InvalidOperationException(
+                    $"SetDllDirectory failed for: {nativeDir}");
+        }
+    }
+
+    [DllImport("kernel32.dll", SetLastError = true, CharSet = CharSet.Unicode)]
+    [return: MarshalAs(UnmanagedType.Bool)]
+    private static extern bool SetDllDirectory(string lpPathName);
+}