Before it sat for many years at a gas station near Baltimore ... Before it stood guard in front of an American Legion hall ... Before it was a yard ornament for the inventor of Bromo-Seltzer ... And before it battled 19th century pirates in the Chesapeake, the Dahlgren 12-pound Light Boat Howitzer was born in the heart of Confederacy at the Tredegar Iron Works in Richmond, Virginia.     After a circuitous journey lasting 143 years, the cannon came home this year to the...