Add godoc to Create and CreateBulk explaining they return immediately
without waiting for block inclusion. Update package-level example to show
both fire-and-forget and wait-for-block patterns. Add Transaction Modes
section to README with code examples.
Enable request timeout and cancellation control by adding context.Context
as the first parameter to all SDK API methods. This allows users to:
- Set per-request timeouts
- Cancel in-flight requests
- Pass request-scoped values
