docs(changelog): prepare v0.3.15 release

[xusd320]

Summary

• Prepare the changelog for the v0.3.15 patch release dated 2026-08-20.
• Capture the two changes merged since v0.3.14: client development middleware/application wrappers and the repository-wide domain reorganization.

Changes

• Add the 0.3.15 release section with feature and improvement notes.
• Leave a new empty Unreleased section for subsequent work.

Validation

• npm run lint — 561 files checked.
• npm run check-types — 31/31 tasks passed.
• npm --workspace evjs-docs run build — English and Chinese builds passed.
• git diff --check — passed.
• Main CI for the release contents passed: https://github.com/afx-team/evjs/actions/runs/32319589755

Risk / rollout

• Documentation-only release preparation; no runtime, configuration, data, security, or performance behavior changes.
• npm publication occurs only after the follow-up v0.3.15 GitHub Release is published.

Reviewer notes

• Verify the version, date, and that the notes cover exactly the changes merged in feat(ev): add client dev middleware and application wrappers #114 and refactor(ev): organize internal build domains #82.