feat(voice): add cache, queue, and worker modules (D-138, Spike 2 Phase 2)

MessagePack voice cache with per-zone persistence and version invalidation.
Priority work queue with crossbeam bounded channel, backpressure, pause/resume,
and zone-change reprioritization. Inference worker pool with empty output guard
and graceful degradation to base text.

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

server/src/voice/mod.rs

@@ -12,4 +12,7 @@
 //! - `worker` — inference worker pool (dynamic scaling, owns sr-voice HTTP clients)
 //! - `hardware` — hardware detection + dynamic sr-voice instance management
 
+pub mod cache;
 pub mod prompt_builder;
+pub mod queue;
+pub mod worker;