[Revinand]

Adds 2.7.0 safe retries. Both features are experimental, off by default and set per resource under idempotency.

What changed

• redelivery: a buyer who lost a paid response retries with the same input and the same retry key (the x402 payment-identifier id or the MPP Idempotency-Key header), and gets the stored response without a second charge.
  • An exact resend of the purchase's proof skips verification until retentionHours ends (1 to 8760, default 24). A fresh proof must verify as the same payer, and it is never settled.
  • Before verification, only the proof that owns a key gets an answer.
  • Key conflicts, in-flight keys and possibly-charged keys get 409 with no new payment challenge. MPP answers a conflict with 422.
• merchantKey: paid backend calls carry a derived Idempotency-Key, which stays the same when a retry is re signed.
• Mapped on HTTP, MCP and A2A. At startup, payment keys left in flight become unresolved, and doctor reports them.
• The receipt database stays at schema v5, and every contract addition is optional.

Why

A response lost after settlement, for example to a dropped connection, left the buyer paying twice.