Prepare 0.4.0 release

[jlc488]

Release prep for v0.4.0 — guardedFetch + sameSitePolicy + lazy node:dns import (#10, site docs #11).

• package.json 0.3.0 → 0.4.0 (publish workflow checks tag ↔ version match)
• CHANGELOG: [Unreleased] → [0.4.0] - 2026-07-13 (GitHub Release body is extracted from this section)
• README maintainer example tag bumped

Release-surface checklist (walked)

1. ✅ org profile README (en+ko) — .github#18: Workers/edge + two-tier guarded fetch description; npm badge auto-updates
2. ✅ repo README (en+ko) — Workers section + guardedFetch/sameSitePolicy (merged docs: Runtime support section — Node vs Cloudflare Workers #9/feat: guardedFetch + sameSitePolicy — runtime-agnostic guarded fetch #10/docs(site): guardedFetch + sameSitePolicy on the landing page #11); no hardcoded version badge
3. ✅ CHANGELOG — [0.4.0] section (this PR)
4. ✅ docs site — landing page updated (merged docs(site): guardedFetch + sameSitePolicy on the landing page #11); version placeholder injects from package.json on deploy
5. N/A GitHub Discussions — repo has none (has_discussions: false); JS releases have no announcement precedent in devslab-examples Discussions
6. N/A devslab-examples — no JS demos; Java demos unaffected

After merge

git tag v0.4.0 && git push origin v0.4.0 → verify → npm publish (provenance) → GitHub Release auto-created.