docs: add implementation plan for CI engine-grouped testing optimization

Plan to restructure the GitHub Actions test matrix from 42 independent jobs
(cfengine x dbengine) to 8 engine-grouped jobs. Each CF engine starts once
and runs all database test suites sequentially, reducing total CI compute
by ~75%.

Key changes planned:
- Engine-only matrix (8 jobs vs 42)
- CFPM install once per Adobe engine instead of 6x
- Oracle sleep 120 replaced with sqlplus health check
- SQL Server memory 4GB->2GB, Oracle 2GB->1.5GB
- JUnit XML reporting via JSON-to-XML conversion
- Engine x database summary grid on workflow runs
- BoxLang volume mounts re-enabled for local dev parity

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

Author: bpamiri