feature, SQL schema backend emitter with PostgreSQL MySQL SQLite3 dialect support

[steven-varga]

Add a SQL DDL backend to h5cpp-compiler that emits CREATE TABLE statements from annotated C++ structs. Support three dialects: PostgreSQL, MySQL, and SQLite3.

Scope

• New backend flag: --sql-postgres, --sql-mysql, --sql-lite3
• Unified output flag model: each backend gets a boolean flag; -o specifies the output file
• Type map: C++ primitives → dialect-specific SQL types
• Nested structs → JSON/JSONB columns (configurable)
• Arrays → dialect-specific array/JSON types
• Constraints: PRIMARY KEY, UNIQUE, NOT NULL, DEFAULT, FOREIGN KEY, CHECK, INDEX
• Attributes: sql::table, sql::column, sql::dialect, sql::primary_key, sql::unique, sql::not_null, sql::default, sql::foreign_key, sql::check, sql::index, sql::type_override, sql::nested

Acceptance Criteria

• SQL DDL emitter (consumer_sql.hpp)
• Attribute rewriter whitelist (h5_attr_translator.hpp)
• CLI flags integrated (h5cpp.cpp)
• Test harness extension (run_fixture.cmake)
• CMake fixtures list updated
• 5+ fixtures with golden baselines
• All tests passing
• Taxonomy document written