Add design and product documentation

DESIGN.md

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+# Design — Volthouse Energy
+
+## Direction
+
+Technical blueprint interface: dark grid environment, hard data panels, luminous system accents, and diagram-led proof.
+
+## System
+
+- Use semantic tokens, tabular numerals, precise units, and consistent diagram grammar.
+- Avoid sci-fi decoration that does not represent system state or flow.
+- Motion may trace energy flow or transitions, with static and reduced-motion alternatives.
+- Mobile reorganizes calculations and diagrams into readable steps rather than scaling them down.
+
+## Intentional patterns
+
+Grid lines, one-pixel dividers, and accent borders are functional blueprint vocabulary and may be explicitly detector-waived.