Stabilize speaker preview boundary test

[r3dbars]

Summary

• keep the exact 0.6-second AVPlayer cutoff assertion
• validate sampled playback stays before the following speaker turn at 0.8 seconds
• remove the hosted-runner-sensitive 20 ms current-time tolerance

Evidence

• focused SpeakerClipPlayback suite: 17/17 passed, three consecutive runs
• full current-main QA before this test-only change: 15/15 passed, including 13,015/13,015 fast tests

This changes test timing only; production playback code is unchanged.