Fix torn notes not combining when moved into inventory

Fixes #8487

Torn notes were only combined when picked up from the ground via
CheckQuestItem(). When notes entered the inventory through other
paths (e.g. stash transfers, item swaps), the combine logic never
ran.

Add CheckSpecialInventoryItem() which checks whether all three torn
notes are present after an item is placed into InvList, and combines
them into the full note if so.

Called from:
- ChangeInvItem (empty slot path)
- ChangeInvItem (swap path)
- AutoPlaceItemInInventory (covers stash-to-inventory transfers)

Author: morfidon

Source/inv.cpp

@@ -498,7 +498,7 @@ bool ChangeInvItem(Player &player, int slot, Size itemSize)
 		if (prevItemId == 0) {
 			player.InvList[player._pNumInv] = player.HoldItem.pop();
 			player._pNumInv++;
-			prevItemId = player._pNumInv;
+			prevItemId = CheckSpecialInventoryItem(player, player._pNumInv - 1) + 1;
 		} else {
 			const int invIndex = prevItemId - 1;
 			if (player.HoldItem._itype == ItemType::Gold)
@@ -512,8 +512,11 @@ bool ChangeInvItem(Player &player, int slot, Size itemSize)
 				if (itemIndex == -prevItemId)
 					itemIndex = 0;
 			}
+			prevItemId = CheckSpecialInventoryItem(player, invIndex) + 1;
 		}
 
+		itemSize = GetInventorySize(player.InvList[prevItemId - 1]);
+
 		AddItemToInvGrid(player, slot - SLOTXY_INV_FIRST, prevItemId, itemSize, &player == MyPlayer);
 	}
 
@@ -1121,6 +1124,49 @@ int CreateGoldItemInInventorySlot(Player &player, int slotIndex, int value)
 
 } // namespace
 
+int CheckSpecialInventoryItem(Player &player, int invIndex)
+{
+	if (invIndex < 0 || invIndex >= player._pNumInv) {
+		return invIndex;
+	}
+
+	const _item_indexes currentId = player.InvList[invIndex].IDidx;
+	const _item_indexes notes[] = { IDI_NOTE1, IDI_NOTE2, IDI_NOTE3 };
+
+	if (IsNoneOf(currentId, IDI_NOTE1, IDI_NOTE2, IDI_NOTE3)) {
+		return invIndex;
+	}
+
+	for (const _item_indexes note : notes) {
+		if (!HasInventoryItemWithId(player, note)) {
+			return invIndex;
+		}
+	}
+
+	player.Say(HeroSpeech::JustWhatIWasLookingFor, 10);
+
+	for (const _item_indexes note : notes) {
+		if (note != currentId) {
+			RemoveInventoryItemById(player, note);
+		}
+	}
+
+	for (int i = 0; i < player._pNumInv; i++) {
+		Item &noteItem = player.InvList[i];
+		if (noteItem.IDidx != currentId) {
+			continue;
+		}
+
+		noteItem = {};
+		GetItemAttrs(noteItem, IDI_FULLNOTE, 16);
+		SetupItem(noteItem);
+		noteItem.updateRequiredStatsCacheForPlayer(player);
+		return i;
+	}
+
+	return invIndex;
+}
+
 void InvDrawSlotBack(const Surface &out, Point targetPosition, Size size, item_quality itemQuality)
 {
 	SDL_Rect srcRect = MakeSdlRect(0, 0, size.width, size.height);
@@ -1400,8 +1446,9 @@ bool AutoPlaceItemInInventory(Player &player, const Item &item, bool sendNetwork
 	if (targetSlot) {
 		player.InvList[player._pNumInv] = item;
 		player._pNumInv++;
+		const int invIndex = CheckSpecialInventoryItem(player, player._pNumInv - 1);
 
-		AddItemToInvGrid(player, *targetSlot, player._pNumInv, itemSize, sendNetworkMessage);
+		AddItemToInvGrid(player, *targetSlot, invIndex + 1, GetInventorySize(player.InvList[invIndex]), sendNetworkMessage);
 		player.CalcScrolls();
 
 		return true;

Source/inv.h

@@ -148,6 +148,15 @@ bool AutoEquip(Player &player, const Item &item, bool persistItem = true, bool s
  */
 bool CanFitItemInInventory(const Player &player, const Item &item);
 
+/**
+ * @brief Handles special item behavior after an item has been inserted into InvList.
+ * @param player The player whose inventory was modified.
+ * @param invIndex Index of the inserted item in InvList.
+ * @return The item's current InvList index. This can change if handling the item
+ * removes other inventory entries and the inventory gets compacted.
+ */
+int CheckSpecialInventoryItem(Player &player, int invIndex);
+
 /**
  * @brief Attempts to place the given item in the specified player's inventory.
  * @param player The player whose inventory will be used.