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/rip.go

@@ -189,7 +189,7 @@ func (g *RogueGame) totalWinner() {
 		}
 
 		g.scr.Std.MvPrintwf(line, 0, "%c) %5d  %s", obj.PackCh, worth,
-			g.invName(obj, false))
+			g.inventoryName(obj, false))
 		line++
 		p.Purse += worth
 	}