grammar: add '<<' shift-left safely (Phase 1); map to AST/MIR/VM; keep '>>' as legacy ARROW

- Tokenizer: add SHIFT_LEFT for '<<' (before <= detection)
- Parser: introduce parse_shift() layer; left-associative; only '<<' handled in Phase 1
- AST: add BinaryOperator::Shl and display
- MIR builder: map Shl -> BinaryOp::Shl
- Interpreter: implement Shl in both execution paths (integer-only, type error otherwise)

No change to '>>' (legacy ARROW remains). No runtime gate yet for SHL as it does not collide with existing syntax.

src/mir/builder/ops.rs

@@ -114,6 +114,7 @@ impl super::MirBuilder {
             BinaryOperator::Multiply => Ok(BinaryOpType::Arithmetic(BinaryOp::Mul)),
             BinaryOperator::Divide => Ok(BinaryOpType::Arithmetic(BinaryOp::Div)),
             BinaryOperator::Modulo => Ok(BinaryOpType::Arithmetic(BinaryOp::Mod)),
+            BinaryOperator::Shl => Ok(BinaryOpType::Arithmetic(BinaryOp::Shl)),
             BinaryOperator::Equal => Ok(BinaryOpType::Comparison(CompareOp::Eq)),
             BinaryOperator::NotEqual => Ok(BinaryOpType::Comparison(CompareOp::Ne)),
             BinaryOperator::Less => Ok(BinaryOpType::Comparison(CompareOp::Lt)),