Design SDK offline transaction preparation model

[Brainoncrack]

Closes #237
Implementation Summary

1. Offline Preparation Module (offline-preparation.ts)
   A complete module for preparing transactions offline with clear network dependencies:

Network Dependencies Identified:

Can be prepared offline: Transaction operations, memos, asset specifications, timebounds, input validation
Requires network state: Source account sequence number (CRITICAL), account balance verification, fee estimation, trustline verification
Core Functions:

prepareTransactionOffline() - Prepare transaction without network access
fetchNetworkState() - Fetch sequence number and account data from Horizon
updateWithNetworkState() - Add sequence number to prepared transaction
buildUnsignedTransaction() - Build unsigned transaction ready for signing
signTransaction() - Sign with secret key (offline-capable)
signTransactionWithSigner() - Sign with Signer interface (hardware wallet support)
submitSignedTransaction() - Submit signed transaction to Horizon
prepareAndSignTransaction() - Complete workflow helper
prepareTransactionWithManualSequence() - Fully offline with manual sequence
2. Type Definitions
Comprehensive type system for offline preparation:

OfflineTransactionParams - Preparation parameters
OfflinePaymentOperation - Payment operation structure
NetworkState - Required network state (sequence number is critical)
PreparedTransaction - Prepared but not yet built
UnsignedTransaction - Built but unsigned
SignedTransaction - Signed and ready for submission
SubmissionResult - Submission outcome
3. Integration
Exported all functions and types from index.ts for public API access.

4. Comprehensive Documentation (offline-transaction-preparation.md)
   Complete guide covering:

Network dependencies (what can be done offline vs. requires network)
Step-by-step workflow with code examples
Use cases: air-gapped signing, multi-party coordination, hardware wallets, transaction queuing
Prepared transaction shapes and type definitions
Signing and submission boundaries
Limitations and considerations (sequence number management, timebounds, fees, trustlines)
Error handling patterns
Safe wrappers for error-safe operations
Security best practices
Key Features
✅ Clear Boundaries - Explicitly separates what can be done offline from what requires network access

✅ Critical Dependency Identification - Sequence number is identified as the critical network dependency

✅ Flexible Workflow - Supports step-by-step preparation or complete workflow helper

✅ Hardware Wallet Support - Signer interface integration for external signers

✅ Manual Sequence Mode - Supports fully offline preparation when sequence number is known

✅ Safe Wrappers - Non-throwing variants for error-safe integration

✅ Comprehensive Error Handling - Clear error codes for missing sequence, account not found, key mismatch, submission failures

✅ Security Considerations - Documentation covers secret key protection, transaction verification, replay protection

Use Cases Enabled
Air-gapped signing: Prepare online, sign offline, submit online
Multi-party coordination: Prepare transaction, collect signatures from multiple parties
Delayed submission: Prepare now, submit when network is available
Transaction queuing: Prepare multiple transactions, submit in batch
Hardware wallet integration: Use external signers for enhanced security