Signature verification module (off-chain approvals)

[nafiuishaaq]

Description:
Allow off-chain signed approvals (EIP-712 style for Soroban) to reduce on-chain transactions.

Acceptance Criteria:

approve_with_signature(session_id, buyer_sig, seller_sig).

Verify signatures using ed25519 verification.

Signature includes session ID and nonce to prevent replay.

Nonce tracked per address.

Emits OffchainApprovalExecuted event.

Labels: integration, signatures, gas-optimization