[SDK · Network & Horizon] Implement StellarSDK main class

[amina69]

Epic: Network & Horizon
Labels: network, types, config, sdk
Size: S (~1 day)

Description

The top-level StellarSDK class that wires all managers together and validates config on init.

Tasks

• Constructor accepts SDKConfig and validates all fields on init
• Validate masterSecretKey format — throw if malformed
• Validate horizonUrl is https:// URL — throw if not
• Instantiate HorizonClient, AccountManager, TransactionManager, EscrowManager
• Expose escrow, accounts, transactions, horizon as public properties
• Export as default export from src/index.ts
• Unit test: valid config creates SDK, invalid key throws, invalid URL throws