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>

docs/potion-stacks.md

@@ -0,0 +1,59 @@
+# Potion counts and potion stacks
+
+## Requests (2026-10-17)
+
+1. The potion selector on the bottom bar always shows how many potions of
+   each kind the hero carries (only when there are any).
+2. Potions of the same type stack in one inventory cell, up to 20 per stack.
+   Every potion entering the inventory tries to stack. Drag, sell and drop
+   move a whole stack. The inventory's sort button stacks first, then sorts.
+   A stack dropped on the ground splits into single potions (agreed for now).
+
+## Measured (diagnostic log, 2026-10-17)
+
+| Action | Calls |
+|---|---|
+| Drink (potion key) | `cEngine::UseActionSlot` `0x613040` → `cCreature::UseItem` `0x54a8f0` → `RemoveItem` (from `0x54aa20`) → `DestroyObject` (from `0x54aa30`); then `UseActionSlot` calls `RemoveItem` again (`0x6130d8`, not found) |
+| Drag in the inventory | `RemoveAt` from `0x6c06fb` (cell → cursor), `PlaceItem` from `0x6c0583` (cursor → cell) |
+| Buy | trader grid (inventory 22) `RemoveItem` `0x6d96f5` → hero grid `PlaceItem` `0x6d98e8` |
+| Pick up from the ground | `PlaceItem` from `0x558db6` |
+| Trader restock | `RemoveItem` `0x6d9973` + `DestroyObject` `0x6d99cc`, new items `PlaceItem` `0x6d9bdf` |
+| Sort | `cInventoryEntry::Sort` `0x5db670`: all items removed (`0x5db8a5`) and placed again (`0x5dbb4c`, notify 0) |
+
+Not seen yet: a sale, a right-click drink.
+
+## Game side
+
+- Potion kinds: `ItemPotionKind` `0x43f630` (ecx = item type table
+  `*0xaa9564`) maps an item type to kind 1..5 (the bottom bar's potion
+  slots); `GetPotionRef` `0x5da880` picks the potion a slot drinks.
+- Bottom bar: `cUI_Taskbar2::RenderPotionSlots` `0x6e39e0` draws the 5 slot
+  controls (embedded at +0x258, stride 0xbc).
+- Inventory grid (`cInventoryEntry`, docs/skill-slots.md): cells of 0xc bytes
+  at +0x18; an item's id only in its first cell, covered cells flagged.
+  `CellRect` `0x5dd530` gives a cell's screen rect (32x32 from the origin
+  +0x1164 / +0x1166, which the window sets before `DrawItems` `0x5dc3f0`).
+- Drinking: `cPotion3D::vf18` `0x5ca9d0` applies the effect and sets object
+  flag +0x14 bit `0x8000000` (used up); `UseItem` then removes and destroys.
+- Sort button: `cUI_Inventory3::HandleEvent` `0x6c5f40`, button control at
+  +0x158, mode from bit 4 of +0x168.
+- Hero files map saved item ids to new objects with `ReplaceItemId`
+  `0x5d80e0`.
+
+## Mods
+
+- `src/mods/potioncounts.cpp`: counts per kind (stacks added up), drawn
+  bottom-right on each potion slot after `RenderPotionSlots`.
+- `src/mods/potionstack.cpp`: stack counts by item id (with the item type),
+  saved as `"count"` on the grid cell in the JSON inventories
+  (`inventory_save.cpp`); entry pours into stacks with room on the next
+  tick; `Sort` stacks the hero's potions first; a drink from a stack of more
+  than one takes one off the count and keeps the object; the count is drawn
+  on the cell after `DrawItems`.
+
+## Open
+
+- Selling a stack: the price is for one potion so far (sale path not measured).
+- Dropping a stack on the ground: it should split into single potions; for now
+  the count stays with the object while the game runs, but a save keeps only
+  the object (one potion).

docs/todo.md

@@ -5,8 +5,8 @@
 Saving marks effect objects (cObjectFX) dead; loading keeps the buff but drops
 the aura effect (type 842) that follows the hero. `src/mods/aura_restore.cpp`
 recreates it after a load for Heldenmut (0x3f7) and Klingenblick (0x3f8). Spell
-auras (Steinhaut, Lichtschild, ...) use other effects and are not covered
-yet; check them if their visuals go missing too.
+auras (Steinhaut, Lichtschild, ...) use other effects; the user confirmed on
+2026-10-17 that their visuals are back after loading too.
 
 ## Data-driven arts instead of hardcoded art ids
 
@@ -125,3 +125,12 @@ in-game texts/tooltips that name the old keys):
 
 Globally bound letters (engine `0x618130`, UI manager `0x757120`): A B C E F
 H I L M N O P Q R S T W, Space, ^, digits. Individual windows may use more.
+
+## Potion stacks
+
+Asked 2026-10-17 (after the potion counts on the bottom bar,
+`src/mods/potioncounts.cpp`): potions of the same type (e.g. minor healing)
+stack in one inventory cell, up to 20 per stack (a full stack starts a new
+one). Drag, sell and drop move the whole stack, as with any other item. The
+counts on the potion slots then add up the stacks. In progress, see
+docs/potion-stacks.md (open: sale price per stack, splitting on the ground).