changelog: add audit log hash chain + JSONL export (May 2026)

Adds two new Improvements entries to the May 2026 section covering the
Day 2 SOC 2 audit chain deliverables that were missing from the public
changelog:
- Audit log hash chain and in-product verification panel
- Audit log JSONL chain export (Enterprise)

Updates the v2026.05 GitHub Release body to include the same entries.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>