docs(system-design): add architecture styles and service boundaries guide

[shalshcode08]

Summary

• add the standalone Topic 12 guide for architecture styles, modular monoliths, microservices, and service boundaries
• compare layered, hexagonal, clean, SOA, event-driven, web-queue-worker, serverless, and microservice constraints
• cover cohesion, coupling, bounded contexts, aggregates, domain and team ownership, database ownership, gateways, BFFs, service discovery, meshes, sidecars, and compatible contracts
• evolve a modular order platform through worker and service extraction with strangler migration, anti-corruption layers, workflow states, and reconciliation
• include production operations, observability, security, performance, testing, failure cases, exercises, interview answers, cheat sheet, and primary official references
• register the page in the System Design navigation manifest

Validation

• npm test
• npm run build
• Prettier check for the topic file and manifest
• git diff --check
• scope check confirms only the topic file and manifest changed
• exact footer check passed
• em dash check passed
• raw code arrow check passed

Notes

• 10,494 words
• preserves existing design classes and adds no styling or test changes
• based directly on origin/main at 82cbb4e
• never updates TOPICS.md before merge