Fix the same 0-based slot assumption in the equipment Lua fallbacks

#2 corrected IsItemInSlot's native path, but the same 0-based assumption was
still present in the two Lua fallbacks, which run on nampower < v2.18. Per
nampower's SCRIPTS.md the slot is 1-19 (16=MainHand, 17=OffHand), and
ClassicAPI's GetInventoryItemID is 1-based as well:

- GetEquippedItem's fallback called GetInventoryItemID(slot + 1), contradicting
  both its own native path and its doc comment.
- GetEquippedItems' fallback built 0-18 keys while the native path returns
  1-19, so the two branches handed back differently-keyed tables.

Neither wrapper currently has any callers, so this was latent rather than
user-visible. Also records the 1-indexed convention at the IsItemInSlot call
site, where the misleading "0-indexed" comment used to be.

NampowerAPI.lua

@@ -1474,7 +1474,7 @@ end
 
 -- Get all equipped items for a unit
 -- Requires v2.18+ for native GetEquippedItems (with Lua fallback for player only)
--- Returns table with slot indices (0-18) as keys, item info tables as values
+-- Returns table with slot indices (1-19) as keys, item info tables as values
 function API.GetEquippedItems(unitToken)
     unitToken = unitToken or "player"
 
@@ -1489,8 +1489,8 @@ function API.GetEquippedItems(unitToken)
     end
 
     local items = {}
-    for slot = 0, 18 do
-        local itemId = CleveRoids.ClassicAPI.GetInventoryItemID("player", slot + 1)
+    for slot = 1, 19 do
+        local itemId = CleveRoids.ClassicAPI.GetInventoryItemID("player", slot)
         if itemId then
             items[slot] = {
                 itemId = itemId,
@@ -1518,7 +1518,7 @@ function API.GetEquippedItem(unitToken, slot)
         return nil
     end
 
-    local itemId = CleveRoids.ClassicAPI.GetInventoryItemID("player", slot + 1)  -- 1-indexed
+    local itemId = CleveRoids.ClassicAPI.GetInventoryItemID("player", slot)  -- both are 1-indexed
     if itemId then
         return {
             itemId = itemId,
@@ -1672,7 +1672,7 @@ function API.IsItemInSlot(itemIdOrName, inventorySlot)
 
     -- Use native v2.18 GetEquippedItem if available
     if GetEquippedItem then
-        local slotInfo = GetEquippedItem("player", inventorySlot)
+        local slotInfo = GetEquippedItem("player", inventorySlot) -- 1-indexed (1-19)
         if slotInfo and slotInfo.itemId then
             local checkId = tonumber(itemIdOrName)
             if checkId then