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>

docs/potion-stacks.md

@@ -57,3 +57,13 @@ Not seen yet: a sale, a right-click drink.
 - 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).
+
+## Chest and other inventories (2026-10-17)
+
+Asked: potions stack in every kind of inventory; traders only sell single
+potions. Measured: the chest is inventory 0x11 (`cUI_Chest` draws window
++0x158), saved with the hero as chunk 0xc6 and in save games, both through the
+JSON path already (stack sizes survived a reload). Stacking now applies to all
+inventories below 0x12 (players and the chest); trader stock (0x12 and up)
+never stacks. Selling a stack to a trader still has to be handled (pay
+count x price, the trader keeps singles).