feat: add MTP support for parallel API

[jhen0409]

Summary

• Add MTP speculative decoding support to the queued parallel completion path.
• Return draft token metrics from parallel completions and update JSI/test coverage.
• Update the MTP example to use the parallel API, configurable slot count, grouped output, and per-completion timing UI.

Validation

• npm run typecheck
• npm run lint
• npm test -- --runInBand
• ./tests/build_and_test.sh
• ./tests/run_tests.sh
• git diff --check