[Refactor] VariableLiteralNode, RuleV2 dataclass, and dead code cleanup

[colinthebomb1]

Summary

• Add VariableLiteralNode AST node for wildcard string literals (e.g. LIKE '%<x>%'), replacing string sentinel handling in
  the parser, rewriter, formatter, and generator
• Update parser to emit ElementVariableNode for variable columns/tables (already in AST, now used at parse time instead of
  post-processing)
• Introduce RuleV2 typed dataclass (core/rule.py) replacing Dict[str, object] rule representations throughout generator and
  data/rules.py
• Dead code removal and simplifications: ~500 lines deleted net across rewriter, generator, and formatter