docs: define Wave 1 turn-taking contract

[rogerchappel]

Summary

• add a Wave 1 turn-taking state machine and audio policy contract for natural voice interactions
• define modes, states, events, timing windows, thresholds, barge-in policy, privacy posture, and VoicePath/AgentPulse integration expectations
• add importable Wave 1 policy/types plus a small invariant test surface and a repo scripts/validate.sh wrapper

Validation

• bash scripts/validate.sh