fix issue where items clustered at the end of belt tiles

src/test/BeltSystemTest.cpp

@@ -226,6 +226,38 @@ TEST_CASE("BeltSystem: item stays blocked when next tile is full", "[belt]")
     REQUIRE(bs.tryTakeItem(eastPort(tileA)).has_value());
 }
 
+TEST_CASE("BeltSystem: belt back slot is capped at progress 0.5", "[belt]")
+{
+    // Use progress/tick = 0.4 so the cap is observable: without it, back would
+    // advance to 0.8 while front is stuck at 1.0, and then need only 1 more tick
+    // after being promoted. With the cap it stays at 0.5 and needs 2 more ticks.
+    const double medBeltSpeed = 0.4 * static_cast<double>(kTickRateHz);
+    BeltSystem bs(medBeltSpeed);
+
+    const QPoint tile(0, 0);
+    bs.placeBelt(tile, Rotation::East);
+
+    // Advance front item to the output edge; it stays there (no next tile).
+    bs.tryPutItem(tile, makeItem("front_item"));
+    bs.tick();  // front: 0.4
+    bs.tick();  // front: 0.8
+    bs.tick();  // front: 1.0 (capped, stuck)
+
+    // Place back item; front is at 1.0 and not blocking (back < 1.0).
+    bs.tryPutItem(tile, makeItem("back_item"));
+    bs.tick();  // back: 0.4
+    bs.tick();  // back would reach 0.8 — must be capped at 0.5
+
+    // Remove front; back (now promoted to front) must be at 0.5, not 0.8.
+    REQUIRE(bs.tryTakeItem(eastPort(tile)).has_value());
+    // At 0.4/tick, 0.5 → 0.9 after one tick — not at 1.0 yet.
+    bs.tick();
+    REQUIRE_FALSE(bs.tryTakeItem(eastPort(tile)).has_value());
+    // 0.9 → 1.0 after a second tick — now available.
+    bs.tick();
+    REQUIRE(bs.tryTakeItem(eastPort(tile)).has_value());
+}
+
 // ---------------------------------------------------------------------------
 // clearTiles
 // ---------------------------------------------------------------------------