Lower Qwen3 speculative accept trace to debug

[xiaguan]

Summary

• Move the per-request DFlash/DSpark speculative acceptance trace from info to debug so default production logging does not emit one line per request per verify round.
• Update the DSpark integration notes to document debug-level acceptance tracing and future structured metrics usage.

Validation

• cargo fmt --check
• git diff --check
• cargo test --release -p openinfer-qwen3-4b --lib
• Pre-commit toxic-reviewer pass: Ready; no remaining DFlash/DSpark production info hot-path trace found.