Add design and product documentation

PRODUCT.md

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+# Product — Universal Landing
+
+Status: product-specific draft required.
+
+## Baseline journey
+
+The page must explain one offer to one primary audience, establish proof, resolve the main objections, and lead to one measurable conversion action.
+
+Before implementation, replace generic placeholders with approved audience, problem, promise, proof, required sections, non-goals, content sources, and conversion criteria.