Potion stacks in every player inventory and the chest, not in trader stock

The chest is inventory 0x11 and already saved as JSON; stacking now applies
to all inventories below 0x12. Traders keep selling single potions.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/game/engine/items.h

@@ -42,7 +42,16 @@ constexpr size_t kInventoryGrid = 0x18;
 constexpr size_t kGridCellSize = 0xc;
 constexpr size_t kGridWidth = 0x115c;   // u16, 0x14
 constexpr size_t kGridHeight = 0x115e;  // u16, 8
-constexpr size_t kInventoryOwner = 0x1160;  // u16: player index (heroes < 0x12)
+constexpr size_t kInventoryOwner = 0x1160;  // u16: = the inventory's index (measured)
+
+// Inventories below 0x12 belong to the players (index = hero id) and the
+// chest (0x11, saved with the hero as chunk 0xc6); PlaceItem / RemoveAt only
+// update the hero below that. Traders use higher ones (22 measured).
+constexpr uint16_t kPlayerInventoryEnd = 0x12;
+constexpr uint16_t kChestInventory = 0x11;
+inline bool IsPlayerInventory(const uint8_t* inventory) {
+    return *reinterpret_cast<const uint16_t*>(inventory + kInventoryOwner) < kPlayerInventoryEnd;
+}
 
 inline unsigned GridCells(const uint8_t* inventory) {
     return *reinterpret_cast<const uint16_t*>(inventory + kGridWidth) *