feat(core): add minecart movement along rail networks

core/include/vf/Minecart.h

@@ -0,0 +1,66 @@
+#pragma once
+
+#include <cstdint>
+#include <functional>
+#include <vector>
+#include "Rail.h"
+#include "Container.h"
+#include "World.h"
+#include "Math.h"
+
+namespace vf {
+
+enum class CartKind : uint8_t {
+    Empty,
+    Chest,
+    Hopper,
+};
+
+constexpr int chestCartSlots = 27;
+constexpr int hopperCartSlots = 5;
+
+struct CartConfig {
+    float maxSpeed = 0.4f;
+    float friction = 0.02f;
+    float slopeAccel = 0.05f;
+    float poweredAccel = 0.06f;
+    float poweredLaunch = 0.06f;
+    float brakeFactor = 0.5f;
+    float stopThreshold = 0.005f;
+    int hopperCooldownTicks = 4;
+};
+
+struct Minecart {
+    Vec3i railPos;
+    RailDir moveDir = RailDir::East;
+    float speed = 0.0f;
+    float offset = 0.0f;
+    CartKind kind = CartKind::Empty;
+    bool derailed = false;
+    int hopperCooldown = 0;
+    Container inventory;
+};
+
+Minecart makeMinecart(const Vec3i& railPos, RailDir moveDir, CartKind kind);
+
+// World-space centre of the cart: the rail cell centre pushed along the heading
+// by the sub-cell offset.
+Vec3 cartWorldPos(const Minecart& cart);
+
+// Advances the cart one simulation tick: applies powered-rail boost or braking,
+// slope gravity and rolling friction, then walks the track by the resulting
+// speed, switching heading through curves and derailing at track ends. Hopper
+// carts also pull one item per cooldown from a container above their cell.
+void tickMinecart(World& world, Minecart& cart, const CartConfig& config,
+                  const std::function<bool(const Vec3i&)>& poweredRailActive,
+                  ContainerStore& store);
+
+// Ticks a batch of carts, then resolves collisions deterministically in index
+// order: two carts that land on the same rail cell exchange momentum (equal
+// mass, elastic 1D) and separate. A head-on pair bounces apart; a moving cart
+// hitting a stalled one hands over its speed and stops.
+void stepMinecarts(World& world, std::vector<Minecart>& carts, const CartConfig& config,
+                   const std::function<bool(const Vec3i&)>& poweredRailActive,
+                   ContainerStore& store);
+
+} // namespace vf