[gmatossian]

Adds a Messaging area under System Design for choosing queues, pub/sub, or retained streams from a concrete requirement. The parent includes a delivery-shapes visual, decision-changing caveats, and a durable-database-jobs alternative.

Five focused children cover message queues, pub/sub, retained logs, delivery/processing safety, and implementations/hosting. The implementation shortlist covers RabbitMQ, ActiveMQ Classic, Kafka, AWS, and GCP, including SQS Standard/FIFO and managed versus self-managed responsibility. All six topics remain draft for later retrieval-based editorial acceptance.

Closes #113.

Decisions and review

• Owner reviewed and approved the representative outline before authoring.
• Used stacked choice/tradeoff blocks after rendered tables hid useful content horizontally. No application rendering changes.
• Kept parked outbox/inbox content, existing Java topics, and unrelated local edits outside this PR.
• AI-assisted authoring, official-source verification, responsive review, and deliberate complete-diff review performed by Codex; no independent review selected. No blocking findings.
• Merge authorized by owner conditional on readiness and successful Actions; editorial acceptance remains separate.

Validation

• npm run check passed: formatting, lint, tool type checks, content generation, 57 app tests, 119 tooling tests, build, and 38 browser tests.
• Reviewed all new children at 390px and the parent at 390px/1280px; no page or content overflow, image loaded and readable.
• git diff --cached --check passed before commit.
• Existing non-failing initial-bundle warning: 631.09 kB against 500 kB warning budget.