Complete v0.4.0 release readiness checklist.

Run through the release checklist:
1. Run automated validation: `./scripts/validate_v0.4.0.sh`
2. Run interactive checklist: `./scripts/release_checklist.sh`
3. Verify all criteria met
4. Review definition of done: `docs/VERSION_0.4.0_DEFINITION_OF_DONE.md`
5. Generate release notes
6. Tag release: `git tag v0.4.0`

Ensure all critical path items are complete before release.

