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>

CMakeLists.txt

@@ -43,6 +43,8 @@ add_library(sacred_re SHARED
     src/mods/input.cpp
     src/mods/skillslots.cpp
     src/mods/inventory_save.cpp
+    src/mods/potioncounts.cpp
+    src/mods/potionstack.cpp
     src/mods/skillbar.cpp
     src/mods/arttarget.cpp
     src/mods/aura_restore.cpp