fix: add explicit panic for Max/Min on empty slices

Adds length checks to Max and Min functions to provide clear,
explicit panic messages when called with empty slices, rather
than allowing confusing index-out-of-bounds panics.

Changes:
- Max: Added check for empty slice with "underscore.Max: empty slice" panic
- Min: Added check for empty slice with "underscore.Min: empty slice" panic
- Updated doc comments to document panic behavior
- Added TestMaxEmpty and TestMinEmpty to verify panic behavior

Impact:
- Better error messages for debugging
- Documented behavior prevents user surprises
- Non-breaking change (still panics, just with clearer message)

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

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

max.go

@@ -3,9 +3,13 @@ package underscore
 import "cmp"
 
 // Max returns the maximum value in the slice.
+// Panics if values is empty.
 // This function can currently only compare numbers reliably.
 // This function uses operator <.
 func Max[T cmp.Ordered](values []T) T {
+	if len(values) == 0 {
+		panic("underscore.Max: empty slice")
+	}
 	max := values[0]
 	for _, v := range values {
 		if v > max {

max_test.go

@@ -13,3 +13,9 @@ func TestMax(t *testing.T) {
 	want := 9
 	assert.Equal(t, want, u.Max(nums))
 }
+
+func TestMaxEmpty(t *testing.T) {
+	assert.Panics(t, func() {
+		u.Max([]int{})
+	})
+}

min.go

@@ -3,9 +3,13 @@ package underscore
 import "cmp"
 
 // Min returns the minimum value in the slice.
+// Panics if values is empty.
 // This function can currently only compare numbers reliably.
 // This function uses operator <.
 func Min[T cmp.Ordered](values []T) T {
+	if len(values) == 0 {
+		panic("underscore.Min: empty slice")
+	}
 	min := values[0]
 	for _, v := range values {
 		if v < min {

min_test.go

@@ -13,3 +13,9 @@ func TestMin(t *testing.T) {
 	want := 1
 	assert.Equal(t, want, u.Min(nums))
 }
+
+func TestMinEmpty(t *testing.T) {
+	assert.Panics(t, func() {
+		u.Min([]int{})
+	})
+}