Rename item-subsystem methods (refactor step 5, items)

getItem→promptPackItem now returns (obj, ok) instead of a nil-signaling
pointer; invName→inventoryName; doPot→applyPotionFuse. C breadcrumbs
kept. Behavior unchanged; suite green.

game/fight.go

@@ -290,7 +290,7 @@ func (g *RogueGame) attack(mp *Monster) int {
 					removed = true
 
 					g.leavePack(steal, false, false)
-					g.msg("she stole %s!", g.invName(steal, true))
+					g.msg("she stole %s!", g.inventoryName(steal, true))
 				}
 			}
 		}