feat(stt): expose Qwen3-ASR token log probabilities

[radraccoon]

Summary

• preserve each selected token log probability in Qwen3-ASR segments
• expose avg_logprob and min_logprob summaries
• cover both sequential and batched chunk decoding
• document the new segment field and add focused tests

The model already computes these distributions and stream_generate() already yields them. The high-level path previously discarded them. This keeps the data available without changing decoded text or sampling behavior.

Closes #984.

Validation

• focused metadata and sequential decode tests pass on Apple Silicon
• black --check passes
• git diff --check passes

[github-actions]

⚠️ The pre-commit checks failed in CI.

Run the following from the repository root, then commit and push any changes:

python -m pip install pre-commit
pre-commit run --all-files

The project's pre-commit configuration includes Black and the other required formatters.

[lucasnewman]

@radraccoon Please run pre-commit run --all to clear the formatter.