Qwen3-ASR streaming timestamps depend on max_tokens instead of audio time

Problem

Qwen3-ASR calculates partial streaming timestamps from token-budget progress:

token_count / remaining_tokens * audio_duration

remaining_tokens starts from the generation budget and changes between chunks. This is not audio time, so changing max_tokens changes timestamps for the same audio and generated text.

Reproduction

Run stream_transcribe() on the same file with max_tokens=64 and max_tokens=8192, then compare each event's start_time and end_time.

Actual: partial timestamps change with the token budget. In one 8.57-second test, 31 partial events covered only about the first 0.03 seconds before the final boundary jumped to the end.

Expected: do not expose token-budget positions as audio timestamps. Text-delta events should be untimed unless the model or an aligner provides real alignment. Coarse audio chunk extents can remain timed.

Tested on current main at 246e233.