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/scrolls.go

@@ -7,8 +7,8 @@ package game
 func (g *RogueGame) readScroll() {
 	p := &g.Player
 
-	obj := g.getItem("read", KindScroll)
-	if obj == nil {
+	obj, ok := g.promptPackItem("read", KindScroll)
+	if !ok {
 		return
 	}