fix(sort): refuse to sort inventory when read is inconsistent with server

Post-sort holes were caused by the planning phase, not the execution
phase. When GetContainerItemInfo/Link transiently returns nil for an
occupied slot during server lag, CreateBagFromID marks it <EMPTY>, the
sort algorithm moves real items into that "destination", and we end up
with literal gaps where the misclassified items used to be.

Validate the read against GetContainerNumFreeSlots before sorting; if
counts disagree, print a chat warning and abort instead of corrupting
the layout. User retries after a moment.

Bagnon/components/sortBtn.lua

@@ -257,6 +257,23 @@ local function SortBag(bag)
 	end
 end
 
+-- Returns true iff every slot in `bagID` whose GetContainerItemLink is nil
+-- is also reported empty by GetContainerNumFreeSlots. When the server is
+-- laggy GetContainerItemInfo/Link can transiently return nil for slots that
+-- actually contain items; sorting on that snapshot generates moves that
+-- treat occupied slots as empty destinations, leaving holes in the result.
+local function IsBagReadConsistent(bagID)
+	local slots = GetContainerNumSlots(bagID) or 0
+	local reportedFree = select(1, GetContainerNumFreeSlots(bagID)) or 0
+	local actualEmpty = 0
+	for s = 1, slots do
+		if not GetContainerItemLink(bagID, s) then
+			actualEmpty = actualEmpty + 1
+		end
+	end
+	return actualEmpty == reportedFree, actualEmpty, reportedFree
+end
+
 local function CreateBagFromID(bagID)
 	local items = GetContainerNumSlots(bagID);
 	local bag = {};
@@ -349,6 +366,18 @@ function SortBtn:OnClick()
 	local bags = {};
 
 	if self.frameID == "inventory" then
+		-- Refuse to sort if any bag's read is inconsistent with the server's
+		-- reported free-slot count — sorting on that snapshot is what causes
+		-- the post-sort holes on a laggy server.
+		for i = 0, NUM_BAG_FRAMES, 1 do
+			local ok, empty, reported = IsBagReadConsistent(i)
+			if not ok then
+				DEFAULT_CHAT_FRAME:AddMessage(format(
+					"|cFFFFAA00Bagnon: bag %d not fully loaded (read %d empty, server says %d) — try sort again in a moment.|r",
+					i, empty, reported))
+				return
+			end
+		end
 		isGuildBankSort = false;
 		for i = 0, NUM_BAG_FRAMES, 1 do
 			local bag = CreateBagFromID(i);