docs(system-design): add messaging and stream processing guide

[shalshcode08]

Summary

• add a complete guide to messages, commands, events, queues, publish-subscribe, and retained logs
• explain partitions, consumer groups, offsets, acknowledgements, delivery guarantees, idempotency, retries, dead-letter handling, replay, and schema evolution
• cover event time, processing time, windows, watermarks, stateful recovery, checkpoints, production operations, security, capacity, testing, interview questions, and a revision cheat sheet
• register the guide in the System Design and Distributed Systems navigation

Why

Topic 10 in the authoritative system-design queue requires a standalone guide that connects messaging mechanics to end-to-end business correctness. The note makes broker guarantees, application effects, failure boundaries, and exactly-once limitations explicit in easy language.

Scope

• src/content/systemdesign/messaging-event-driven-stream-processing.html
• manifest.json

No styling, shared design, tests, or queue statuses were changed.

Validation

• Prettier formatting passed
• complete 22-test suite passed
• 40-page production build passed
• HTML, manifest, orphan, internal-link, anchor, landmark, and design-token checks passed
• exact scope and git diff --check passed
• required footer and no-em-dash checks passed