Feature: Add audio playback service (stream WAV to robot speaker) #34

Summary

Support sending audio clips to Vector via HA service.

Why this matters

Advanced capability; lower priority than core settings/actions.

SDK evidence (no wire-pod dependency)

• Bidirectional stream RPC: ExternalAudioStreamPlayback
• Messages: ExternalAudioStreamPrepare, ExternalAudioStreamChunk, ExternalAudioStreamComplete

Proposed HA design

• Service: vector.play_audio
• Input: media source with explicit format constraints

Acceptance criteria

• Supported formats/sample rates documented.
• Streaming includes strict timeouts and robust errors.
• Tests for validation and transport failure paths.