docs(changelog): prepare v0.3.21 release

[xusd320]

Summary

• Prepare v0.3.21 with Application component mode and explicit HTTP method middleware composition.

Changes

• Move the current unreleased changelog entries under the dated 0.3.21 section and leave Unreleased ready for subsequent changes.

Validation

• npm run lint
• npm run check-types
• npm --workspace evjs-docs run build (English and Chinese)
• git diff --check

Risk / rollout

• This PR changes release metadata only.
• The release includes removal of automatic API directory middleware and relocation of middleware exports; migration requirements are documented in the changelog.
• Publish through the existing GitHub Release workflow after merge. The workflow sets all package versions and internal dependency versions to 0.3.21.

Reviewer notes

• Confirm the release version, date, and breaking-change guidance.

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-03T11:11:30.661614Z	d521d52	PR opened

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