docs(architecture): sync docs with actual codebase structure

[bella-247]

Sync the architecture documentation to match the actual codebase:

Directory tree

• Add internal/semantic/ (6 new files)
• Add internal/validator/ (validator + tests)
• Add cmd/synthgraph-web/ (server/ subdirectory with 10 files)
• Add cmd/serveviz/
• Remove non-existent internal/cli/

Pipeline diagram

• Add Semantic Analysis stage between Graph Builder and Topo Sort
• Add Post-generation Validator stage after Generator

Dependency table

• Add semantic row
• Fix generator row: also imports graph and semantic

Error types

• Add PostGenValidationError (validator package)
• Fix format string arg count mismatch

Co-Authored-By: Claude noreply@anthropic.com

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