Stabilize AWS Neptune module

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### Overview:

Amazon Neptune is a fast, reliable, fully managed graph database service that makes it easy to build and run applications that work with highly connected datasets. The core of Neptune is a purpose-built, high-performance graph database engine. This engine is optimized for storing billions of relationships and querying the graph with milliseconds latency. Neptune supports the popular graph query languages Apache TinkerPop Gremlin and W3C’s SPARQL, enabling you to build queries that efficiently navigate highly connected datasets. Neptune powers graph use cases such as recommendation engines, fraud detection, knowledge graphs, drug discovery, and network security.

[AWS Docs](https://docs.aws.amazon.com/neptune/latest/userguide/intro.html)

### Maturity: CloudFormation Resources Only

See the [AWS Construct Library Module Lifecycle doc](https://github.com/aws/aws-cdk-rfcs/blob/master/text/0107-construct-library-module-lifecycle.md) for more information about maturity levels.

### Implementation:

See the [CDK API Reference](https://docs.aws.amazon.com/cdk/api/latest/docs/aws-neptune-readme.html) for more implementation details.

### Issue list:

Start with the linked AWS Neptune documentation and CDK API reference to understand the existing CloudFormation-only module and its intended maturity path. The issue names no source files, tests, constructs, or concrete acceptance criteria; scope would need to be defined through the missing implementation checklist and related issue or pull request links before work can be considered complete.

Written by the indexing model from the issue text.