feat(voice): add hardware detection + dynamic sr-voice instance management (Phase 4)

GPU-aware scaling: NVIDIA (nvidia-smi), AMD (sysfs VRAM), Apple Silicon
(unified memory). GPU mode detected at install, persisted to settings.
Scaling ceiling: (free_resource - existing_llm_usage) / 2 / per_instance_cost.
VoiceInstanceManager spawns/stops sr-voice processes on unique ports.
Battery detection scales to 1 worker.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/voice/mod.rs

@@ -13,6 +13,7 @@
 //! - `hardware` — hardware detection + dynamic sr-voice instance management
 
 pub mod cache;
+pub mod hardware;
 pub mod lookup;
 pub mod prompt_builder;
 pub mod queue;