Add Python SQLModel code generator design document

DOCS: Add comprehensive design for Python SQLModel/Pydantic codegen

This document outlines the complete design for implementing a Python
code generator that converts PostgreSQL DDL schema definitions to
SQLModel and Pydantic models. Key aspects include:

- Architecture with modular components (type mapping, naming, models)
- Comprehensive PostgreSQL to Python type mapping table
- Field generation patterns for all constraint types
- Edge case handling (reserved words, circular refs, arrays, JSONB)
- Testing strategy with unit and snapshot tests
- Implementation plan broken into 8 phases

Also adds genco 0.19 as a dependency for Python code generation.

https://claude.ai/code/session_01Mr4TzZKNorjrdtJyVnDwuW

Author: claude

Cargo.lock

@@ -4,4 +4,5 @@ version = "0.1.0"
 edition = "2024"
 
 [dependencies]
+genco = "0.19.0"
 tern-ddl = { path = "../ddl" }