[JFWooten4]

Adds an opt-in Soroban mainnet anchor to the existing SEP-53 PDF signing flow without putting the PDF or signer identity on-chain.

• adds a minimal proof-anchor Soroban contract that accepts a 32-byte digest, emits it with the ledger sequence/timestamp, and keeps no contract storage
• derives the anchored digest deterministically from the canonical SEP-53 message plus the finalized base64 signature
• adds a deployment-owned relayer path using Stellar RPC simulation/assembly/submission on public mainnet
• hides the anchor option unless both the contract ID and relayer secret are configured
• records the contract ID, transaction hash, ledger, digest, and server confirmation time with the public proof
• links anchored proofs to their Stellar mainnet transaction while leaving the normal signing path off-chain
• documents contract build/deployment and the dedicated relayer configuration

The relayer key is separate from the document signer: users still sign only the SEP-53 message through their wallet, and the server never receives their wallet secret.