Add design and product documentation

PRODUCT.md

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+# Product — Juniper Table
+
+Status: approved baseline.
+
+## Audience and job
+
+Diners and event hosts need to understand the seasonal menu, service windows, seating formats, allergy handling, and booking confirmation process.
+
+## Primary journeys
+
+1. Explore current menu, sourcing, pairing, and dining format.
+2. Choose a valid date, time, party size, and seating option.
+3. Disclose relevant allergies or request private dining and understand host confirmation.
+
+## Non-goals
+
+- No guaranteed availability, hidden service terms, or unsupported instant confirmation.
+
+## Success criteria
+
+Menu and booking information stay current and legible, constraints are visible before submission, and private dining has a distinct qualified path.