Animate items emerging from building output ports

src/lib/sim/BeltSlot.h

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+#pragma once
+
+#include <vector>
+
+#include <QPoint>
+#include <QPointF>
+
+#include "Item.h"
+#include "Rotation.h"
+
+// A single item on a belt-like lane: an item plus its fractional progress along
+// the lane's travel direction. Shared by BeltSystem's belt/tunnel tiles and by a
+// building's virtual output belt (REQ-MAT-OUTPUT-EMERGE) so the packing and
+// geometry live in exactly one place.
+struct BeltItemSlot
+{
+    Item   item;
+    double progress;  // [0.0, 1.0]: 0 = just entered, 1 = at output edge
+};
+
+// Advances every slot in `slots` by `progressPerTick`, applying the standard belt
+// packing: the front (index 0) carries the highest progress; each following slot
+// stays at least 0.25 behind the slot ahead and is capped at 1.0 - i * 0.25.
+// `slots` must be ordered front (highest progress) first. This is the per-tile
+// advance shared by belts, tunnel entries, and tunnel exits.
+void advanceBeltSlots(std::vector<BeltItemSlot>& slots, double progressPerTick);
+
+// World-space centre (in tile units) of a slot at `progress` on a lane occupying
+// `tile` and flowing in `dir`. Progress 0 = entry edge, 1 = output edge.
+QPointF beltSlotWorldPos(QPoint tile, Rotation dir, double progress);