Digest replay protection in @suimpp/mpp server verification

[funkiirabu]

Problem

The @suimpp/mpp server verify() callback checks the transaction on-chain (succeeded, right recipient, right amount) but does not track used digests. The same TX digest could be presented against multiple challenges — paying once but consuming multiple API calls.

Context

mppx already solves this for the tempo method with assertHashUnused / markHashUsed + a store interface. The same pattern should be added to the sui method.

Proposed Solution

Add digest tracking to @suimpp/mpp/server:

1. Add an optional store interface to SuiServerOptions:

   interface DigestStore {
     has(digest: string): Promise<boolean>;
     set(digest: string): Promise<void>;
   }

2. In verify(), before returning the receipt:

   • Call store.has(digest) — if true, throw "Digest already used"
   • Call store.set(digest) — mark as consumed

3. Provide a default in-memory store (Map with TTL) for single-instance deployments

4. Allow custom stores (Redis, DB) for multi-instance / serverless deployments

References

• mppx tempo implementation: assertHashUnused / markHashUsed pattern
• Current verify: packages/mpp/src/server.ts lines 39-119