Initial scaffold: docs, data manifest, and static dashboard

docs/data-structure.md

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+# Data Structure
+
+## Data Sources
+- CSV exports (source of truth)
+- DAX exports (measure definitions)
+
+## Storage
+- `/data/*.csv` — raw CSV exports
+- `/data/manifest.json` — list of CSV files loaded by the app
+
+## Notes
+- No database yet; app loads CSVs directly via fetch.
+- Column types and relationships will be documented here once data lands.