feat: add wallet ffi function for arbitrary public transactions

[adklempner]

Purpose

Add a generic FFI function for sending arbitrary public transactions to any deployed program, removing the need to add per-program FFI wrappers.

Related: #422

Approach

• Add wallet_ffi_send_public_transaction C FFI function that accepts a program ID, account list, raw instruction bytes, and a signer
• Add WalletCore::send_public_transaction async method that builds, signs, and submits a PublicTransaction
• Add InvalidArgument error variant for input validation (e.g. instruction length not a multiple of 4)
• Generate updated C header (wallet_ffi.h)

How to Test

1. Build the wallet-ffi crate: cargo build -p wallet-ffi
2. Call wallet_ffi_send_public_transaction from a logos-core module (see companion PR in logos-execution-zone-module)
3. Verify the transaction is accepted by the sequencer and state is updated

Dependencies

None.

Future Work

• Support arbitrary private (shielded) transactions via FFI (the other half of [FFI] support arbitrary program data in transactions #422)

PR Completion Checklist

• Complete PR description
• Implement the core functionality
• Add/update tests
• Add/update documentation and inline comments