docs: define VAD on first use in ARCHITECTURE.md

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

Author: chazmaniandinkle

ARCHITECTURE.md

@@ -80,7 +80,7 @@ class ModalityModule(ABC):
     health() -> dict                # Diagnostics
 ```
 
-`Gate` is optional. Text has no gate (all text passes). Voice uses VAD to reject silence.
+`Gate` is optional. Text has no gate (all text passes). Voice uses VAD (Voice Activity Detection) to reject silence.
 
 ## The Bus (bus.py)