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/potioncounts.cpp

@@ -0,0 +1,73 @@
+// Potion counts on the potion slots of the bottom bar.
+//
+// Request (2026-10-17): the potion selector always shows how many potions of
+// each kind the hero carries (only when there are any).
+//
+// The bar shows 5 potion kinds: cUI_Taskbar2::RenderPotionSlots (0x6e39e0)
+// draws control k-1 (embedded at +0x258, stride 0xbc) when
+// cInventoryEntry::GetPotionRef(k) finds a potion of kind k, else the empty
+// control behind it (+0x3ac). GetPotionRef walks the inventory grid and asks
+// ItemPotionKind for each item's kind; the count here does the same and adds
+// up the potion stacks (potionstack.cpp).
+
+#include <cstdint>
+#include <cwchar>
+
+#include "game/engine/items.h"
+#include "game/engine/objects.h"
+#include "game/ui/text.h"
+#include "mods/potionstack.h"
+#include "mods/skillslots.h"
+#include "re/hook.h"
+
+namespace mods::potioncounts {
+namespace {
+
+// cUI_Taskbar2 fields.
+constexpr size_t kInventoryIndex = 0x158;  // u16: the hero's inventory
+constexpr size_t kPotionSlots = 0x258;     // game::kPotionKinds embedded controls
+constexpr size_t kPotionSlotSize = 0xbc;
+
+constexpr uint16_t kCountFont = 1;  // as the art cooldown labels (skillbar.cpp)
+constexpr uint32_t kCountColor = 0xffffffff;
+constexpr int kCountInset = 3;  // from the slot's bottom-right corner
+
+void RenderCounts(uint8_t* taskbar, void* renderer) {
+    const uint8_t* inventory =
+        skillslots::Inventory(*reinterpret_cast<const uint16_t*>(taskbar + kInventoryIndex));
+    if (!inventory || !game::GetPlayerHero()) {
+        return;
+    }
+    unsigned counts[game::kPotionKinds + 1] = {};
+    game::ForEachGridItem(inventory, [&](uint32_t id) {
+        const int kind = game::PotionKindOf(id);
+        if (kind >= 1 && kind <= game::kPotionKinds) {
+            counts[kind] += potionstack::StackCount(id);
+        }
+    });
+    const int lineHeight = game::FontLineHeight(kCountFont);
+    for (int kind = 1; kind <= game::kPotionKinds; ++kind) {
+        if (counts[kind] == 0) {
+            continue;
+        }
+        game::UiRect rect = game::ControlScreenRect(taskbar + kPotionSlots + (kind - 1) * kPotionSlotSize);
+        rect.y += rect.h - lineHeight - kCountInset;
+        rect.w = static_cast<int16_t>(rect.w - kCountInset);
+        wchar_t text[8];
+        swprintf_s(text, L"%u", counts[kind]);
+        game::DrawText(renderer, kCountFont, rect, text, kCountColor, game::kTextRight);
+    }
+}
+
+} // namespace
+} // namespace mods::potioncounts
+
+// cUI_Taskbar2::RenderPotionSlots (0x6e39e0, ret 4): then the counts.
+void __fastcall UiTaskbar_RenderPotionSlots(void* self, void* edx, void* renderer);
+RE_HOOK(0x006e39e0, UiTaskbar_RenderPotionSlots);
+void __fastcall UiTaskbar_RenderPotionSlots(void* self, void* edx, void* renderer) {
+    UiTaskbar_RenderPotionSlots_original(self, edx, renderer);
+    if (self) {
+        mods::potioncounts::RenderCounts(static_cast<uint8_t*>(self), renderer);
+    }
+}