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.Core/AudioChunk.cs

@@ -0,0 +1,3 @@
+namespace Robovoice.Core;
+
+public sealed record AudioChunk(float[] Samples, int SampleRate);