Preview-mode gate

Normal-tier (free, unpaid) users get EVERY calculator in full and a limited, read-mostly preview of the Model Builder — enough to evaluate the flagship apps' shape before a subscription unlocks full editing and solving.

• A prospect wants to try before they buy, without a sales call
• You're checking what an un-upgraded account actually sees on a gated page
• You're deciding how much of a new flagship-app surface a preview should show

In the flagship apps the same page renders for every role, but write/solve actions are gated behind a role check — a normal-tier user sees the layout and inputs, but the Run/Solve action redirects to upgrade instead of executing. Calculators carry no such gate: they compute in full for everyone.