feat: add FoldRight function

- Add FoldRight: fold/reduce from right to left
- Useful for non-associative operations
- Comprehensive tests including comparison with Reduce
- Benchmark included

Example: FoldRight([1,2,3], 0, subtract) → 1-(2-(3-0)) = 2

Resolves Issue 20

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

foldright.go

@@ -0,0 +1,13 @@
+package underscore
+
+// FoldRight is like Reduce but processes elements from right to left.
+// Also known as foldr in Haskell.
+//
+// Example: FoldRight([]int{1,2,3}, 0, func(n, acc int) int { return n - acc })
+//   → 1 - (2 - (3 - 0)) = 1 - (2 - 3) = 1 - (-1) = 2
+func FoldRight[T, P any](values []T, acc P, fn func(T, P) P) P {
+	for i := len(values) - 1; i >= 0; i-- {
+		acc = fn(values[i], acc)
+	}
+	return acc
+}

foldright_test.go

@@ -0,0 +1,80 @@
+package underscore_test
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+
+	u "github.com/rjNemo/underscore"
+)
+
+func TestFoldRight(t *testing.T) {
+	nums := []int{1, 2, 3, 4}
+	result := u.FoldRight(nums, 0, func(n, acc int) int { return n + acc })
+	assert.Equal(t, 10, result)
+}
+
+func TestFoldRightEmpty(t *testing.T) {
+	result := u.FoldRight([]int{}, 42, func(n, acc int) int { return n + acc })
+	assert.Equal(t, 42, result)
+}
+
+func TestFoldRightSingleElement(t *testing.T) {
+	result := u.FoldRight([]int{5}, 0, func(n, acc int) int { return n + acc })
+	assert.Equal(t, 5, result)
+}
+
+func TestFoldRightSubtraction(t *testing.T) {
+	// FoldRight: 1 - (2 - (3 - 0)) = 1 - (2 - 3) = 1 - (-1) = 2
+	nums := []int{1, 2, 3}
+	result := u.FoldRight(nums, 0, func(n, acc int) int { return n - acc })
+	assert.Equal(t, 2, result)
+}
+
+func TestFoldRightDivision(t *testing.T) {
+	// FoldRight with float: 2.0 / (4.0 / (8.0 / 1.0)) = 2.0 / (4.0 / 8.0) = 2.0 / 0.5 = 4.0
+	nums := []float64{2.0, 4.0, 8.0}
+	result := u.FoldRight(nums, 1.0, func(n, acc float64) float64 { return n / acc })
+	assert.Equal(t, 4.0, result)
+}
+
+func TestFoldRightStrings(t *testing.T) {
+	words := []string{"a", "b", "c"}
+	result := u.FoldRight(words, "", func(s, acc string) string { return s + acc })
+	assert.Equal(t, "abc", result)
+}
+
+func TestFoldRightVsReduce(t *testing.T) {
+	nums := []int{1, 2, 3}
+
+	// Reduce (left fold): (0 - 1) - 2 - 3 = -6
+	reduceResult := u.Reduce(nums, func(n, acc int) int { return acc - n }, 0)
+	assert.Equal(t, -6, reduceResult)
+
+	// FoldRight: 1 - (2 - (3 - 0)) = 1 - (2 - 3) = 1 - (-1) = 2
+	foldRightResult := u.FoldRight(nums, 0, func(n, acc int) int { return n - acc })
+	assert.Equal(t, 2, foldRightResult)
+
+	// They should be different for non-associative operations
+	assert.NotEqual(t, reduceResult, foldRightResult)
+}
+
+func TestFoldRightBuildList(t *testing.T) {
+	nums := []int{1, 2, 3}
+	result := u.FoldRight(nums, []int{}, func(n int, acc []int) []int {
+		return append([]int{n}, acc...)
+	})
+	assert.Equal(t, []int{1, 2, 3}, result)
+}
+
+func BenchmarkFoldRight(b *testing.B) {
+	nums := make([]int, 1000)
+	for i := range nums {
+		nums[i] = i
+	}
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		u.FoldRight(nums, 0, func(n, acc int) int { return n + acc })
+	}
+}