Add text-to-speech via Piper with --audio flag and replay verb

Enable TTS with `h2g2 --audio` to hear game text spoken aloud using
the Piper en_GB-alan-medium voice. Audio plays in a background thread
while text displays normally. The new `replay` in-game verb replays
the last spoken audio. Voice model path is configurable via --voice.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

pyproject.toml

@@ -4,7 +4,9 @@ version = "0.1.0"
 description = "The Hitchhiker's Guide to the Galaxy - Python text adventure engine"
 readme = "README.md"
 requires-python = ">=3.12"
-dependencies = []
+dependencies = [
+    "piper-tts>=1.2.0",
+]
 
 [build-system]
 requires = ["hatchling"]