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/interpreter/expressions/operators.rs

@@ -323,6 +323,19 @@ impl NyashInterpreter {
                 }
             }
             
+            BinaryOperator::Shl => {
+                // Integer-only left shift
+                if let (Some(li), Some(ri)) = (
+                    crate::runtime::semantics::coerce_to_i64(left_val.as_ref()),
+                    crate::runtime::semantics::coerce_to_i64(right_val.as_ref()),
+                ) {
+                    return Ok(Box::new(IntegerBox::new(li.wrapping_shl(ri as u32))));
+                }
+                Err(RuntimeError::TypeError { 
+                    message: format!("Shift-left '<<' requires integers (got {} and {})", left_val.type_name(), right_val.type_name()) 
+                })
+            }
+            
             BinaryOperator::Less => {
                 let result = CompareBox::less(left_val.as_ref(), right_val.as_ref());
                 Ok(Box::new(result))

src/interpreter/operators.rs

@@ -256,6 +256,17 @@ impl NyashInterpreter {
                     Ok(Box::new(BoolBox::new(self.is_truthy(right_val))))
                 }
             },
+            
+            BinaryOperator::Shl => {
+                if let (Some(li), Some(ri)) = (
+                    left_val.as_any().downcast_ref::<IntegerBox>(),
+                    right_val.as_any().downcast_ref::<IntegerBox>(),
+                ) {
+                    Ok(Box::new(IntegerBox::new(li.value.wrapping_shl(ri.value as u32))))
+                } else {
+                    Err(RuntimeError::InvalidOperation { message: format!("Shift-left '<<' requires integers (got {} and {})", left_val.type_name(), right_val.type_name()) })
+                }
+            },
         }
     }