Extract a standalone Rust transaction builder crate from the CLI

[tomerweller]

Summary

Every Stellar SDK (Python, JS, Java, Go) provides a high-level TransactionBuilder for constructing, signing, and submitting transactions. Rust has no equivalent — developers must work directly with stellar-xdr types, which is low-level and error-prone.

The CLI already contains transaction-building logic (builder traits, operation construction, signing, simulation/assembly), but it's tightly coupled to CLI concerns (clap, config, printing). Extracting this into a standalone crate would fill the gap and give Rust developers a first-class transaction building experience.

What exists today in the CLI

• TxExt trait for building transactions (tx/builder/transaction.rs) — already has no CLI dependencies
• Amount and Asset helpers (tx/builder/)
• Assembled struct for post-simulation transactions (assembled.rs) — depends only on stellar-xdr + soroban-rpc
• Signer abstraction with local key, Ledger, Lab, and secure store backends (signer/)
• 23 operation builders (commands/tx/new/) — currently coupled to clap/config
• Soroban authorization signing — mostly pure crypto

Proposed scope

A new workspace crate (e.g. cmd/crates/stellar-transaction-builder/) providing:

• TransactionBuilder with a fluent API matching other SDKs: source account, fee, sequence number, operations, memo, preconditions, build()
• Operation helper functions for all 23 operation types (pure functions: XDR types in, OperationBody out)
• TransactionSigner trait decoupled from CLI printing/config
• Assembled transaction wrapper and simulation support (feature-gated behind rpc)
• Soroban auth signing utilities

What stays in the CLI

• sim_sign_and_send_tx orchestrator (caching, printing, explorer URLs)
• clap argument parsing and config/address resolution
• User interaction (prompts, confirmations)

Prior art

• Python: stellar_sdk.TransactionBuilder — fluent method chaining, 60+ operation append methods
• JS: StellarSdk.TransactionBuilder — constructor with options, addOperation(), build()
• Java: org.stellar.sdk.TransactionBuilder — builder pattern with method chaining
• Go: txnbuild.NewTransaction(TransactionParams{...}) — struct-based params with operation array

All follow the same separation: building (pure) → simulation (async) → signing (crypto) → submission (async).

Suggested approach

1. Create the new crate alongside existing code (no breaking changes to CLI)
2. Migrate CLI to consume the new crate incrementally
3. stellar-ledger already demonstrates this extraction pattern successfully