feat: Phase 4 final demo, eval summary, and report

[AD2000X]

feat: Phase 4 final demo - eval summary + report

Phase 4 - final integration

Aggregates the per-phase evaluation artifacts into one summary, a written report,
and an artifact-backed Gradio final demo. Assembles existing metrics - no new research
(GriTS / Ragas / DeepEval are future work).

Contents

• src/phase4_summary.py, scripts/build_phase4_summary.py, tests/test_phase4_summary.py
• reports/phase4_metrics.md, reports/final_report.md, notebooks/07_final_report.ipynb
• scripts/run_demo.py, notebooks/06_demo.ipynb
• docs/phase4_brief.md; README/DEVLOG/PLAN status refresh

Verification

• pytest 246 passed
• reports/phase4_metrics.md no-drift on rebuild
• demo imports without importing gradio at module import time
• degraded path works with no API key / no embedding stack
• outputs/ and data/ stay gitignored; committed report artifacts stay under reports/