Refactor 03_recursive sum (Java) to use int[]

[luciannamend]

"You’re given an array of numbers. You have to add up all the numbers and return the total."

Replaced ArrayList with a primitive int[] in the recursive sum method to improve performance and reduce memory usage.

• Faster execution: Avoids boxing/unboxing and uses direct array access.
• Lower memory usage: Primitive arrays are lighter than ArrayList with boxed types.
• Simpler code: No need for imports or list management for fixed-size data. This version is ideal for static numeric datasets where performance and clarity are priorities.