Design JavaParser-backed source scanning

[JuanTorchia]

Summary

• Add a design note for the next source scanning layer using JavaParser.
• Identify STRUCTURED_CONCURRENCY_PREVIEW as the first AST-backed rule candidate.
• Map AST evidence back to the existing SourcePattern and Finding flow.
• Document why symbol solving, automatic rewrites, type resolution, and plugin APIs stay out of the first implementation.
• Link the design from the rule contribution guide.

Validation

• python -B scripts/check-local-markdown-links.py
• Legacy documentation path search returns only the historical rename note in docs/engineering-log/012-public-repository-readiness.md.
• Public documentation hygiene search returns no workstation-specific paths or private planning references.
• git diff --check
• mvn clean test '-Dmaven.compiler.release=22'

Closes #4.