[proton0210]

Expose aws:cdk:propertyAssignment metadata as AssemblyIndexNode.propertyMutationStacks, grouped by CloudFormation property name. Preserve every recorded trace and make it available through both construct-path and template-location lookups, including nested stacks and assemblies.

Fixes #2064.

This PR targets explorer_2_assembly_loading and depends on #2055, which introduces AssemblyIndex. The diff contains only the mutation-site follow-up.

Validation:

• Schema jsii compilation and assembly API TypeScript compilation pass.
• All 41 assembly-index tests pass, including repeated traces, missing debug metadata, prototype-like property names, nested stacks and nested assemblies. The new mutation cases fail against the unmodified base implementation.
• Full assembly API suite: 196/197 tests pass; all 8 snapshots pass. The unchanged test assembly with tree metadata having no file property specified also fails on base commit e61caab: it expects Invalid assembly manifest, while the base throws Invalid TreeCloudArtifact. Missing "file" property.
• Focused ESLint and git diff --check pass.

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[maintainer-for-aws]

Automated review

A maintainer will still review this — treat the notes below as a starting point.

📌 Commit f031958

This PR surfaces property-mutation traces on AssemblyIndex nodes in the @aws-cdk/cloud-assembly-api package, addressing a feature request to query mutation-site metadata that aws-cdk-lib emits in debug mode. It adds an optional propertyMutationStacks field on AssemblyIndexNode (keyed by CloudFormation property name) and fills in the previously-reserved PROPERTY_ASSIGNMENT metadata case, along with README documentation and tests.

The change is purely additive, with no modification to existing exports or behavior. The accumulation logic uses a null-prototype map to neutralize prototype-pollution from user-controlled property names, preserves traces in metadata order, and the destructured entry shape matches the schema's PropertyMutationMetadataEntry. Test coverage is thorough, including prototype-like names, repeated and empty traces, and reachability through both byPath() and byTemplateLocation() for nested stacks and assemblies. No issues found on this change.

🔴 0 blocking · 🟡 0 recommended · ⚪ 0 optional

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