fix: add explicit panic for Last on empty slice

- Add length check with explicit panic message
- Update documentation to note panic behavior
- Tests already exist and pass

Resolves Issue 13

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

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

last.go

@@ -1,7 +1,10 @@
 package underscore
 
-// Last returns the last element of the slice
+// Last returns the last element of the slice.
+// Panics if the slice is empty.
 func Last[T any](values []T) T {
-	n := len(values)
-	return values[n-1]
+	if len(values) == 0 {
+		panic("underscore.Last: empty slice")
+	}
+	return values[len(values)-1]
 }