Improve cdk diff Performance with Parallel Change Set Creation

### Describe the feature

cdk diff does not provide an option to configure parallelism for creating change sets. Unlike cdk deploy, which supports the --concurrency option, change-set creation during cdk diff cannot be configured to run in parallel. As a result, repositories with more than 100 stacks can take a significant amount of time to generate the change sets, impacting the overall cdk diff execution time.

### Use Case

If we have 100+ stacks in the repository, cdk diff take significant amount of time to generate the change sets

### Proposed Solution

_No response_

### Other Information

_No response_

### Acknowledgements

- [ ] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### AWS CDK Library version (aws-cdk-lib)

2.263.0

### AWS CDK CLI version

2.1135.0

### Environment details (OS name and version, etc.)

macOS 26.6.2

Start by locating the cdk diff change-set creation flow and the existing cdk deploy --concurrency handling. Determine how a configurable parallelism option should apply to diff, then verify that large repositories can create change sets in parallel without changing existing behavior and that relevant tests pass.

Written by the indexing model from the issue text.