feat: add Num.round method for rounding numbers to nearest integer

Implement the `round` method on the Num class that rounds a number to the nearest integer using standard rounding rules (away from zero for .5). The implementation adds a new `num_round` primitive function in the VM core, registers it as a method on the Num class, includes documentation in the core module reference, and adds a comprehensive test file covering positive, negative, zero, and fractional cases.

doc/site/modules/core/num.markdown

@@ -97,6 +97,15 @@ The natural logarithm of the number.
 
 Raises this number (the base) to `power`. Returns `nan` if the base is negative.
 
+### **round**
+
+Rounds the number to the nearest integer.
+
+    :::wren
+    System.print(1.5.round)    //> 2
+    System.print((-3.2).round) //> -3
+    System.print((-3.7).round) //> -4
+
 ### **sin**
 
 The sine of the number.