list what a belt is carrying, and clear it by name

The belt card's only content was a button reading "Clear stuck items", which
assumed a state the items need not be in. It now reads "Clear items", and above
it the card lists what the selected tiles hold -- one item chip per type, the
same chip the buffer sections and the HQ's block stock draw -- so a line's
contents can be read before they are removed, and can be read at all: items on a
moving belt are too small and too transient to count by eye, and items inside a
tunnel are drawn nowhere.

BeltSystem gains countItems(tiles), a query of the same kind as
forEachVisualItem: a method rather than exposed tile containers, so the per-tile
representation stays swappable. It walks the same five containers as clearTiles,
in the same order, so the list and the button cannot drift apart.

The tunnel's two ends are now told apart. Items in transit are counted on the
exit they are travelling toward, and a clear removes exactly what the panel
listed for the tile it acts on: the exit discards them, the entry leaves them
travelling. BeltSystemTest's tunnel case splits in two accordingly.

Splitters now aggregate with belts and tunnel ends. Their output filters are
per-object configuration, which an aggregate simply does not show -- a splitter
selected alone still gets them. Both cards share one BeltItemList widget, and
both derive their tiles from collectBeltTiles, so nothing is stated twice.

The mixed count summary loses the clear action it carried: a button acting on
part of a selection is worse than no button, and the tiles can be selected by
themselves.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ne3mejABZoLWKLh8fgpM3x

src/lib/sim/BeltSystem.h

@@ -95,13 +95,26 @@ public:
     double getProgressPerTick_tpt() const { return m_progressPerTick_tpt; }
 
     // -- Maintenance ---------------------------------------------------------
-    void clearTiles(const std::vector<QPoint>& tiles);   // REQ-UI-BELT-CLEAR
+    // Removes every item from the given tiles (REQ-UI-BELT-CLEAR). A tunnel exit takes
+    // the items in transit through its tunnel with it; a tunnel entry does not, so what
+    // a clear removes is exactly what countItems reported for the same tile.
+    void clearTiles(const std::vector<QPoint>& tiles);
     void tick();
 
     // -- Rendering -----------------------------------------------------------
     void forEachVisualItem(QRect viewportTiles,
                            std::function<void(VisualItem)> visit) const;
 
+    // -- Inspection ----------------------------------------------------------
+    // What the given tiles carry, summed per item type (REQ-UI-BELT-ITEMS). Items in
+    // transit through a tunnel are counted on its exit, that being where they will
+    // arrive and the only end whose clear discards them (REQ-BLD-TUNNEL-TRANSIT).
+    //
+    // A method rather than an exposed container: what a tile holds and how it holds it
+    // is this subsystem's alone to know, so the per-tile representation stays swappable
+    // (architecture.md, Belt Subsystem).
+    std::map<ItemType, int> countItems(const std::vector<QPoint>& tiles) const;
+
     // -- Determinism ---------------------------------------------------------
     // Folds all transport state (belt/splitter/tunnel tiles and their items)
     // into the hasher in deterministic order (see docs/replay_design.md).