Add design and product documentation

DESIGN.md

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+# Design — Common Ground
+
+## Direction
+
+Editorial newsroom meets public ledger: human stories, documentary imagery, visible evidence, and restrained civic color.
+
+## System
+
+- Use semantic tokens, editorial type hierarchy, and asymmetric but readable grids.
+- Keep documents, budgets, and dates visually distinct from narrative content.
+- Avoid card grids as the default; use rhythm, rules, captions, and section composition.
+- Motion supports story progression only and must not obscure evidence.
+- Mobile retains captions, provenance, and action context.
+
+## Intentional patterns
+
+Hairlines, index numbers, and side rules are intentional editorial navigation devices when consistently applied.