feat(arch): initialize specialized documentation for hexagonal-architecture and cross-links

[beginwebdev2002]

• Created a modular documentation structure for Hexagonal Architecture under ./architectures/hexagonal-architecture/.
• Decomposed content into readme.md, data-flow.md, folder-structure.md, trade-offs.md, and implementation-guide.md.
• Implemented sequenceDiagram, graph TD, and classDiagram using Mermaid with correct styling limits.
• Established strict boundaries and Vibe Coding LLM/Agent code constraints.
• Updated root architectures/readme.md to map to the new specialized files for FSD, MVC, and Hexagonal Architecture.

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PR created automatically by Jules for task 12149046710914812609 started by @beginwebdev2002

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