Add mpp.land registry entry to discovery.mdx

[etal37]

What

Adds mpp.land to the registries table in the discovery documentation.

Why

mpp.land is an open registry that continuously verifies MPP services against
the spec. It scans every indexed service every 5 minutes and publishes
compliance scores, uptime history, and spec validation results.

Discovery documents are advisory (as the docs note). A service can publish a
valid-looking spec and behave differently at runtime. mpp.land catches these
discrepancies by probing the 402 challenge flow, validating response schemas
against declared specs, and tracking changes over time.

What it provides:

• Compliance scoring — every service is graded A–F against the MPP spec
  (TLS, challenge binding, digest params, 402 declarations, schema completeness)
• Uptime monitoring — discovery and challenge reachability checked every
  5 minutes with response time tracking
• Version history — API changes, grade changes, and uptime transitions
  logged in a public changelog
• LLM-friendly endpoints — llms.txt, llms-full.txt, and structured JSON
  so agents can query the registry programmatically
• Budget protection layer — use.mpp.land (coming soon) adds budget
  enforcement, response previews, and reliability routing as a free proxy

[vercel]

@etal37 is attempting to deploy a commit to the Tempo Team on Vercel.

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