## 1. Implementation
- [x] Keep Microsoft MAS package/assets available without an Azure AEC runtime feature flag.
- [x] Capture and buffer microphone/system input streams separately before mixing.
- [x] Clean microphone chunks with a local echo cancellation stage before mixing.
- [x] Mix cleaned microphone and system audio by addition with configurable gains.
- [x] Keep temporary recording output to the mixed WAV; do not persist diagnostic channel sidecars.
- [x] Add focused tests for local echo cleanup, additive mixing, aligned mixing, and mixed-only temporary recording.

## 2. Verification
- [x] Run focused tests.
- [x] Run `dotnet test MeetingAssistant.slnx`.
- [x] Run `openspec validate add-audio-headroom-and-azure-aec --strict`.