MAHUBAH'S CORNER, by Avalyn Hunter <p>Most veteran campaigners at the racetrack are geldings. So a mare that proves hardy enough to run in stakes company year after year is something truly special. Such is the case with Silmaril, who retired from racing Jan. 12 after winning the What a Summer Stakes at Laurel Park. The daughter of Diamond was a stakes winner for five consecutive years and retired as Maryland's 17th equine millionaire. Now, her future as a broodmare is wide open.