fix: VAD time mapping timestamp drift caused by overlap samples (#3711)

fix: VAD time mapping timestamp drift caused by overlap samples (#3711)

* whisper : fix VAD segment overlap boundary handling

 - Use original segment length (pre-overlap) for vad_end in the time
   mapping table, so segment boundaries are preserved accurately

Claude Sonnet 4.6 (Low)

* whisper : remove intermediate VAD time mapping points

Now that segment boundaries are mapped accurately, the intermediate
point interpolation is no longer necessary.

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Co-authored-by: Lohopupa <redacted>