Add design and product documentation

PRODUCT.md

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+# Product — Volthouse Energy
+
+Status: approved baseline.
+
+## Audience and job
+
+Technical and commercial decision-makers need to understand energy systems, quantify capacity and resilience, inspect delivered projects, and request an engineering assessment.
+
+## Primary journeys
+
+1. Match a facility problem to an engineering solution.
+2. Inspect calculations, diagrams, case evidence, maintenance, and SLA.
+3. Submit site, load, timing, and constraint data for a qualified assessment.
+
+## Non-goals
+
+- No invented savings, unsupported efficiency claims, or ornamental engineering diagrams.
+
+## Success criteria
+
+Technical evidence is understandable, calculators expose assumptions, and assessment requests carry enough data for a useful response.