Summary

Add a measured advisory sync benchmark note to the README and update the freshness section to reflect the current offline DB behavior.

Closes #87.

What Changed

• added a local benchmark callout to the offline advisory DB sync workflow
• scoped the benchmark specifically to advisory DB sync on the same machine and OSV npm dump
• noted that results vary by machine and network conditions
• updated the advisory DB freshness section to reflect that freshness reporting and stale warnings already exist

Benchmark Data

• before: 87.53s real
• after: 8.84s real
• improvement: about 9.9x faster end-to-end

Verification

• docs-only content review