Add Datalog evaluator and type resolution packages

Introduce two new self-contained packages:

- internal/datalog: A purpose-built Datalog evaluator implementing
  semi-naïve bottom-up evaluation with stratified negation. Supports
  symbol interning, rule building, nested-loop joins, and fixpoint
  computation (~720 lines).

- internal/datalog/typeres: SQL type resolution expressed as Datalog
  rules over relations (base_type, implicit_cast, operator, function_sig,
  expr_type, etc.). Derives resolved_type, castable, and common_type.
  Includes LoadPostgreSQLTypes() with ~25 common PG types and their
  cast paths (~680 lines).

https://claude.ai/code/session_01Tp4qzqnkRGVxJnJa9ARtDi

Author: claude