Deep Merge

Implement solve(target, source) that recursively merges source into
target and returns the merged result.

Rules

• When both values are plain objects, merge recursively.
• Otherwise, source value overwrites target value.
• Don't mutate the inputs — return a new object.

Example

solve({ a: 1, b: { c: 2, d: 3 } }, { b: { c: 99, e: 4 }, f: 5 })
// → { a: 1, b: { c: 99, d: 3, e: 4 }, f: 5 }