[Test logic] Metadata progress drift formula is never exercised

[balloob]

Problem

The spec's "Calculating current track position" section provides an explicit drift formula using timestamp, progress.track_progress, and progress.playback_speed:

calculated_progress = progress.track_progress + (current_time - metadata.timestamp) * progress.playback_speed / 1000000

The metadata scenario (server-initiated-metadata) sends track_progress=12000, track_duration=180000, playback_speed=1000 and compares a static snapshot — equality pass/fail, no time advancement.

Proposed fix

Add a scenario that:

1. Sends metadata at server time T with track_progress=12000, playback_speed=1500 (1.5x).
2. Holds the connection open for ~500 ms.
3. Requests the client report its computed current_track_progress_ms at disconnect time.
4. Asserts it is within tolerance of 12000 + 500 * 1.5 = 12750.

Also cover playback_speed=0 (paused — calculated progress must equal track_progress unchanged) and verify min/max clamping around track_duration and zero.

Spec references

• "Calculating current track position"