interpreter: implement List indexing operations

[Prestgg1]

Goal

Implement list indexing (read and write) in the interpreter.

Current State

The parser supports Expr::Index { target, index, target_type } and generates the correct AST. However:

1. The validator passes Expr::Index through but converts it to ValidatorExpr::BinaryOp with dummy values, losing the original index semantics:

   ParserExpr::Index { target, index, target_type } => {
       Ok(ValidatorExpr::BinaryOp {
           left: Box::new(target),
           right: Box::new(index),
           op: Operation::Equal,  // wrong - this is not an equality check
           return_type: target_type,
       })
   }

   This corrupts the AST and makes indexing impossible to execute correctly.

2. The runner has no Expr::Index variant (it was removed from validator AST).

3. Need to add Index variant to validator::ast::Expr and properly implement it in both validator and runner.