Soroban Contract Event Log Subscriber

[Kingsman-99]

Description

Soroban smart contracts emit structured events that the SDK consumes but only processes synchronously during transaction result parsing. Long-running processes need to subscribe to a contract's event stream and react to new events (e.g. split_registered, payment_released) as they are emitted on-chain, without polling for each transaction individually. A contract event log subscriber should abstract the getEvents RPC call into a reactive subscription.

Technical Context

Add src/contractEventSubscriber.ts. Uses @stellar/stellar-sdk SorobanRpc.Server.getEvents() with filters: [{ type: 'contract', contractIds: [...] }]. Polls at a configurable interval (default 5 s) and yields only new events since the last startLedger. Adds ContractEventFilter and ParsedContractEvent to src/types.ts. Integrates with src/events.ts and src/cursorTracker.ts for ledger position persistence.

Acceptance Criteria

• ContractEventSubscriber.subscribe(contractId, filter): AsyncIterableIterator<ParsedContractEvent> yields new contract events as they appear on-chain
• Parses event topics and data from XDR using @stellar/stellar-sdk xdr.ScVal decode utilities
• Persists the last-processed ledgerSequence via src/cursorTracker.ts so the subscription resumes correctly after a restart
• unsubscribe() stops the polling loop and releases resources
• Unit tests mock SorobanRpc.getEvents() responses across three poll cycles and assert that only new events (not duplicates) are yielded
• All CI checks (npm test, npm run build, ESLint/TypeScript) pass and the branch has no merge conflicts

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