feat(core): introducing CDK Mixins

[mrgrain]

Reason for this change

Mixins enable composable, reusable functionality that attaches to constructs without inheritance. A mixin author implements supports() and applyTo(), and consumers apply mixins to any scope using Mixins.of(scope).apply(myMixin). Construct selectors narrow the target set by resource type, path pattern, or ID glob. This composability is the key differentiator from Aspects: where Aspects visit every node in the tree, Mixins give authors fine-grained control over which constructs receive which behavior.

Description of changes

The core Mixins types (Mixins, Mixin, IMixin, MixinApplicator, ConstructSelector, and related interfaces) now live in @aws-cdk-lib/core/lib. The MIXIN_SYMBOL identifier changes from the preview value to @aws-cdk/core.Mixin, which deliberately breaks cross-version compatibility checks so that preview mixins and GA mixins remain distinct.

The @aws-cdk/mixins-preview package re-exports these types from aws-cdk-lib/core instead of bundling its own copies. Existing code that imports from @aws-cdk/mixins-preview continues to work without changes. The preview package keeps its additional features like built-in mixins and EventBridge event patterns.

A new "Mixins" section in the aws-cdk-lib README documents the Mixins.of() entry point, custom mixin creation, construct selection, and error handling with requireAny() / requireAll(). It sits right before the existing "Aspects" section because the two features are conceptually related.

Analytics metadata tracks mixin usage via aws:cdk:analytics:mixin node metadata. The FQN of first-party mixins (those with allowed package prefixes) is recorded; third-party or unidentified mixins are redacted to *.

Describe any new or updated permissions being added

N/A

Description of how you validated changes

aws-cdk-lib compiles cleanly with tsc --noEmit. Unit tests cover the Mixin base class, MixinApplicator (including requireAny/requireAll semantics and ordering guarantees), ConstructSelector variants, and metadata recording with FQN redaction.

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

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