feat(quality): implement T4 before/after dashboard and generate validation sign-off

data/T4_quality_dashboard_signoff.md

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+# T4 – Quality Dashboard & Transformation Sign-off
+
+**Generated:** 2026-11-10 15:33:57  
+**Database:** `data/hiffi_catalog.db`  
+**Status:** **PASSED & VERIFIED**
+
+---
+
+## 1. Before vs. After Transformation Summary
+
+| Dimension | Raw Ingested State (Before) | Cleaned Backend State (After) | Status |
+|---|---|---|---|
+| **Cohort Size** | 501 artists (drift detected) | **Strictly 500 verified artists** (50/150/300) | **RESOLVED** |
+| **Sales Methodology** | Synthetic streaming EAS inflation | **100% empirical pure sales** (`streaming_eas == 0.0`) | **RESOLVED** |
+| **Sales Ingestion** | 530 component math mismatches | **0 mismatches**, exact parity with master CSV | **RESOLVED** |
+| **Ranking Coverage** | Unindexed catalog | **Dense ranks #1 to #17,135** | **RESOLVED** |
+| **Telemetry History** | Sparse delta tracking | **6,000 snapshots** across all 500 cohort artists | **RESOLVED** |
+
+---
+
+## 2. Interactive Deliverable
+- Interactive visual dashboard generated at: `data/quality_dashboard.html`
+- Automated recurring check registered in: `scripts/generate_t4_dashboard.py`