Add the transparent PromptSpend Receipt

[AndrewAvery7]

What this adds

• a real /receipt/ product route with a premium copyable Receipt object
• one visible, temporary instruction contract backed by current PromptSpend pricing
• generated /receipt/spec.json and /receipt/instructions.txt artifacts
• request-by-request observed-session cost arithmetic in the shared core
• accessibility, responsive, outage, safety, discovery, and regression coverage

Verification

pm run verify`n-
pm run test:e2e (136 passed, 4 intentional viewport skips)