New backend 'qwen3-mlx-simul' for Apple Silicon: AlignAtt border
detection via monkey-patched cross-attention on MLX Qwen3-ASR.
Supports 0.6B (RTF 0.236 on M5) and 1.7B models.
- qwen3_mlx_simul.py: full streaming implementation with KV cache,
alignment head attention extraction, border-distance policy
- core.py: register new backend in TranscriptionEngine + online_factory
- parse_args.py: add qwen3-mlx-simul to CLI choices
