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/potionstack.h

@@ -0,0 +1,19 @@
+#pragma once
+
+#include <cstdint>
+
+namespace mods::potionstack {
+
+// Potions of one type in the stack held by item `id` (1 for any other item).
+unsigned StackCount(uint32_t id);
+
+// Sets the stack size of item `id` (1 or less: no stack). Used when loading.
+void SetStackCount(uint32_t id, unsigned count);
+
+// cInventoryEntry::ReplaceItemId: the stack follows its item to the new id.
+void OnItemIdReplaced(uint32_t oldId, uint32_t newId);
+
+// Forgets all stacks (a save game is about to be loaded).
+void ResetStacks();
+
+} // namespace mods::potionstack