Add SDK destination account validation strategy

[Brainoncrack]

Closes #241
Implementation Summary

1. Destination Validation Module (destination-validation.ts)
   A complete validation module with clear local/network boundaries:

Local Validation (synchronous, no network calls):

Address format validation (G... public key shape)
Address checksum validation (Ed25519 verification)
Self-payment detection
Asset specification format validation
Network Validation (asynchronous, Horizon queries):

Account existence check (funded status)
Account status verification (active/inactive)
Trustline verification for issued assets
Trustline authorization status
Trustline capacity checks (when amount provided)
2. Validation Functions
validateDestinationLocal() - Fast synchronous validation
validateDestinationNetwork() - Comprehensive network validation
validateDestinationComplete() - Combined validation with level control
validateDestinationOrThrow() - Throwing helper for integration
Safe wrappers: safeValidateDestination(), safeValidateDestinationLocal(), safeValidateDestinationNetwork()
3. Type Definitions
DestinationValidationLevel - 'local' | 'network' | 'complete'
DestinationValidationStatus - 11 status codes for different outcomes
DestinationValidationOptions - Configurable validation options
DestinationValidationResult - Structured result with metadata
4. Integration
Exported all validation functions and types from index.ts for public API access.

5. Comprehensive Tests (destination-validation.test.ts)
   Test coverage includes:

Local validation scenarios (valid/invalid addresses, self-payment, asset formats)
Network validation scenarios (account existence, trustline checks, capacity limits)
Complete validation with level control
Error handling and edge cases
Safe wrapper behavior
6. Documentation (destination-validation.md)
Complete guide covering:

Validation strategy overview
Usage examples for all functions
Validation status codes reference
Error handling patterns
Best practices
Integration with payment flows
Key Features
✅ Clear Boundaries - Local validation is fast and synchronous; network validation is comprehensive but requires Horizon queries

✅ Flexible Levels - Choose 'local', 'network', or 'complete' validation based on your use case

✅ Rich Metadata - Results include sequence numbers, balances, trustline limits, and capacity information

✅ Error Codes - Machine-readable error codes for programmatic handling

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

✅ Trustline Support - Full support for issued asset trustline validation including authorization and capacity checks