The VoiceConfig struct only. Its defaults, normalise/validate arms and
UseLLMRouter follow in the next commit, because the pre-commit cap counts a
pure move at twice the block size.
The RouterThreshold doc drifted: it said the default was 0.35 and that 0.0
meant permissive. DefaultRouterThreshold is 0.55 and applyDefaults replaces
anything <= 0, so permissive was never reachable. Fixed the comment, not the
code.
