docs(child-5): add final testing report — all stages complete

All 7 stages of Child #5 Integration Testing passed:
- Stage 2: E2E full deploy success (44s, all 6 URLs → 200 OK)
- Stage 3: repository_dispatch from all 4 products verified
- Stage 5: Build 44-61s, page load ~240ms — well within targets
- Stage 6: Concurrency group correct, unknown events ignored

Production readiness: CONFIRMED

Closes: Child #5 testing phase
Related: #7, #2

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: InfoTech.io Bot