feat: typed Soroban contract integration layer

[TomikeDS]

Description

Adds a typed Soroban contract integration layer to src/lib/stellar.ts with Zod runtime validation, exponential backoff retry, request timeout, and structured error classes.

Closes #13

Key Changes

• Typed contract methods: verifyDocument, getDocumentStatus, registerDocument, revokeDocument — each with Zod schema validation of RPC responses
• Retry with exponential backoff: 3 retries on transient (5xx) errors, no retry on 4xx or validation failures
• Request timeout: Configurable per-call timeout (default 30s) with AbortController cancellation
• Error classes: SorobanRpcError and SorobanTimeoutError with structured codes/statuses
• Zod schemas: verificationResultSchema, documentStatusSchema, registerResultSchema, revokeResultSchema for runtime response validation

Testing

• 23 unit tests covering error classes, Zod schemas, RPC call behavior, retry logic, schema validation, typed contract methods, and service properties
• npm run test — all new tests pass (2 pre-existing failures in api-validation.test.ts unrelated to this change)
• npm run build — only pre-existing import.meta.vitest type error in errorHandler.ts

Type of Change

• New feature (non-breaking change which adds functionality)