bug(mixer): barge-in has audible gap — woman stops before man starts

[shaypal5]

Problem

At ~1:19 in sp_it_a_0001_00, the man is supposed to cut the woman off mid-sentence (BARGE_IN mix mode). Instead, the woman audibly stops speaking BEFORE the man starts, with a noticeable time gap. This sounds like two people politely taking turns, not an interruption.

Source: Listening test, 2026-05-03.

Expected behavior

In a barge-in:

1. Woman is speaking
2. Man's turn audio begins while woman is still audible
3. Woman's audio is truncated shortly after the overlap starts

Likely cause

The SceneMixer BARGE_IN logic may be:

• Truncating the woman's turn at the planned onset of the man's turn, rather than letting them overlap
• Adding inter-turn silence/gap even in BARGE_IN mode
• Or the script generator may not be producing BARGE_IN mix mode annotations for this turn pair

Investigation needed

1. Check the MixedScene turn onset/offset values for the relevant turns — is there overlap in the timestamps?
2. Check if the turn's mix_mode is actually BARGE_IN or defaulting to SEQUENTIAL
3. Review SceneMixer.mix_barge_in() for how it handles the truncation point

Impact

P2. Affects realism of escalation scenes but doesn't block other work.