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/tables.go

@@ -94,7 +94,7 @@ type gameData struct {
 	initWeaps [NumWeaponTypes]weaponSetup
 
 	// pActions is potions.c p_actions[]. The P_SEEINVIS message is dynamic
-	// (it names the fruit) and is computed in doPot.
+	// (it names the fruit) and is computed in applyPotionFuse.
 	pActions [NumPotionTypes]pact
 
 	// idType maps identify scrolls to the kind of item they identify