A fond farewell to Pam Cross, the longtime WCVB-TV (Channel 5) anchor, who announced her resignation Tuesday. Cross, who has been with the station for 35 years, said her last day will be March 5. In a letter to colleagues, Cross wrote: “WCVB has given me a window on the world to see and chronicle caring, chaos, cruelty and creativity. There’s no way to set a value on what I’ve seen, heard and learned. . . . This is my opportunity to pull the shade on this window and open the door to new challenges.’’