feat(core): add player inventory and hotbar slots

core/include/vf/Inventory.h

@@ -0,0 +1,70 @@
+#pragma once
+
+#include <array>
+#include "Block.h"
+
+namespace vf {
+
+constexpr int hotbarSize = 9;
+constexpr int stackLimit = 99;
+
+struct ItemStack {
+    Block block = Block::Air;
+    int count = 0;
+    bool empty() const { return count <= 0 || block == Block::Air; }
+};
+
+// The player's hotbar: a fixed row of stacks with one selected slot. Placing a
+// block consumes from the selected stack; breaking a block returns it here.
+class Hotbar {
+public:
+    Hotbar() {
+        const Block starters[hotbarSize] = {Block::Grass, Block::Dirt,   Block::Stone,
+                                            Block::Sand,   Block::Wood,   Block::Leaves,
+                                            Block::Glowstone, Block::Water, Block::Snow};
+        for (int i = 0; i < hotbarSize; ++i) slots[i] = {starters[i], 64};
+    }
+
+    int selectedIndex() const { return selected; }
+    Block selectedBlock() const { return slots[selected].block; }
+    const ItemStack& slot(int index) const { return slots[index]; }
+
+    void select(int index) {
+        selected = ((index % hotbarSize) + hotbarSize) % hotbarSize;
+    }
+
+    // Wheel or bracket keys: step the selection, wrapping both ends.
+    void scroll(int steps) { select(selected + steps); }
+
+    // Breaking a block: stack it onto a matching slot, else the first empty one.
+    void add(Block block, int count) {
+        if (block == Block::Air || count <= 0) return;
+        for (ItemStack& stack : slots) {
+            if (stack.block == block && stack.count < stackLimit) {
+                stack.count += count;
+                if (stack.count > stackLimit) stack.count = stackLimit;
+                return;
+            }
+        }
+        for (ItemStack& stack : slots) {
+            if (stack.empty()) {
+                stack = {block, count > stackLimit ? stackLimit : count};
+                return;
+            }
+        }
+    }
+
+    // Placing a block: draw one from the selected stack. False if it was empty.
+    bool consumeSelected() {
+        ItemStack& stack = slots[selected];
+        if (stack.empty()) return false;
+        if (--stack.count <= 0) stack.count = 0;
+        return true;
+    }
+
+private:
+    std::array<ItemStack, hotbarSize> slots;
+    int selected = 0;
+};
+
+} // namespace vf