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/Cargo.toml

@@ -18,13 +18,16 @@ thiserror = "2"
 tracing = "0.1"
 tracing-subscriber = { version = "0.3", features = ["env-filter", "json"] }
 clap = { version = "4", features = ["derive"] }
+crossbeam-channel = "0.5"
+ureq = { version = "2", features = ["json"] }
+sysinfo = "0.35"
+serde_json = "1"
 
 [features]
 default = ["gauntlet"]
 gauntlet = []
 
 [dev-dependencies]
-serde_json = "1"
 
 # ---------------------------------------------------------------------------
 # Explicit test target for the Layer 3 integration module (D-030, ticket #200).