docs(changelog): record integration follow-ups

[piotr-roslaniec]

Summary

• list the integration and assurance follow-up PRs composing the unreleased hardening set
• document bounded proof-input behavior and ModProof context reuse
• record constant-time Paillier performance work and the exact mixed-binary harness scope

Verification

• git diff --check origin/dev...HEAD
• confirmed the PR diff is CHANGELOG.md only

[piotr-roslaniec]

Final evidence at 9771cfe286e05ba1c90f068cc461fbd92e2105ac: the diff is CHANGELOG.md only, git diff --check passed, Go-fmt run 36691643368 passed, and full Go Test run 36691643226 passed.