Potion counts on the bottom bar and potion stacks in the inventory

- potioncounts.cpp: each potion slot shows how many potions of its kind the
  hero carries (stacks added up)
- potionstack.cpp: potions of one type stack up to 20 per cell; every
  potion entering the hero's grid pours into stacks with room, the sort
  button stacks first, drinking takes one off a stack, counts drawn on the
  cells and saved in the JSON inventories
- game/engine/items.h: potion kinds and grid helpers; ObjectManager and
  DestroyObject in objects.h
- auto-attack: the projectile-weapon target radius is a design value
- docs/potion-stacks.md: measured paths and the design

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

src/mods/skillslots.cpp

@@ -13,6 +13,7 @@
 // Network events sent from the shadow would name slot - 5: multiplayer with
 // more than 5 slots is not supported.
 
+#include "mods/potionstack.h"
 #include "mods/skillslots.h"
 
 #include <cstring>
@@ -202,6 +203,7 @@ void __fastcall Inv_ReplaceItemId(void* self, void* edx, int oldId, int newId);
 RE_HOOK(0x005d80e0, Inv_ReplaceItemId);
 void __fastcall Inv_ReplaceItemId(void* self, void*, int oldId, int newId) {
     WithShadow(Inv_ReplaceItemId_original, self, oldId, newId);
+    mods::potionstack::OnItemIdReplaced(static_cast<uint32_t>(oldId), static_cast<uint32_t>(newId));
 }
 
 void __fastcall Inv_FixInventory(void* self, void* edx);