fix: stabilize realtime playback transitions and transcript scrolling

[torbys]

Summary

• prevent stale playback chunks and command feedback from being misclassified during runtime transitions
• preserve transcript scrolling beneath the voice stage
• document the assistant playback race investigation and remediation direction

Fixes #329

[github-actions]

Go coverage

Scope	Current	Baseline	Delta
packages/contracts	89.6%	89.6%	+0.0%
services/api	64.0%	64.0%	+0.0%
services/realtime-audio	82.5%	82.6%	-0.1%
Total	74.0%	74.0%	+0.0%

Base branch: dev · Commit: 08cd0f7 · Test workflow run

[fennoai]

Reviewed the playback lifecycle, mode-switch cleanup, command interruption classification, and voice-stage hit testing. The focused realtime packages and race-enabled playback/pipeline/runtime tests pass. One remaining concurrency issue is called out inline: the new availability wait does not reserve the physical track, so independently started outputs can still race after the wait and lose their first chunk.

[jinyu918]

IGTM

[Gwen317]

The targeted playback-owner fix addresses the original mode-switch interruption race, but one P1 remains: phrase playback cleanup still clears the whole scheduler queue after the mode commit. A new-mode Turn admitted in that interval is marked superseded and its audio is dropped. Please fence cleanup to pre-switch work or serialize phrase admission with the transition.

All current GitHub checks are green, but I am not approving until this race is fixed.

[Gwen317]

已确认此前的问题均已处理：playback reservation 竞态、模式切换后的 owner 定向中断、切换后新 phrase 任务保留，以及主动取消任务不再被错误重试。最新 realtime race tests 和 GitHub CI 均通过，approve。