Add Forge Cascade Checkout Discovery API

[SunFlash12]

Adds Forge Cascade Checkout Discovery to the public API collection.

Evidence

• Public OpenAPI: https://forgecascade.org/openapi.json
• Public enterprise checkout/discovery page: https://forgecascade.org/enterprise
• Public buyer route: https://forgecascade.org/buy
• Public LLM discovery: https://forgecascade.org/llms.txt
• Checkout discovery JSON: https://forgecascade.org/buy.json
• Offer catalog JSON: https://forgecascade.org/offers.json
• MCP metadata: https://forgecascade.org/.well-known/mcp.json

Validation Performed On Submission Branch

• npm ci
• focused schema parse of collection/forge-cascade-checkout-discovery.yaml against dist/schema.js: passed
• npm run build: passed and generated the README/page entries

Route-First Buyer Verification

Verified June 5, 2026 UTC: Forge buyer and proof routing is route-first on the production domain. Production Forge routes preserve attribution before Stripe handoff; the proof pack records the remaining MCP fallback field pending production redeploy.