Implement scrap pile selection

Scrap piles become a third selection category: click to select (actors
win on overlap), box-drag / Ctrl+click multi-select, and the selected-
object panel shows the summed remaining amount, updating live and
dropping piles as they are collected or despawn.

- ScrapSystem: expose per-pile amount via ScrapInfo.
- EntityHitTest: add scrapAtWorldPos and scrapInBox (with tests).
- New header-only ScrapSelectionChangedEvent.
- GameWorldView: scrap selection member, click/box handling with the
  building-wins disambiguation, and per-frame pruning of gone piles.
- SelectedBuildingPanel: scrap event, "Scrap: N" label, live total;
  scrap and building/entity selections clear each other.

Implements REQ-UI-SCRAP-CLICK-SELECT, REQ-UI-SCRAP-MULTI-SELECT,
REQ-UI-SCRAP-PANEL.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DZR44tA8sn4dPqDzAVXyps

src/lib/eventsystem/event/ScrapSelectionChangedEvent.h

@@ -0,0 +1,18 @@
+#pragma once
+
+#include <vector>
+
+#include "entt/entity/entity.hpp"
+
+#include "Event.h"
+
+// The set of currently selected scrap piles (REQ-UI-SCRAP-CLICK-SELECT,
+// REQ-UI-SCRAP-MULTI-SELECT). An empty list means no scrap is selected. Scrap forms
+// its own selection category, mutually exclusive with buildings and entities.
+class ScrapSelectionChangedEvent : public Event
+{
+public:
+    explicit ScrapSelectionChangedEvent(std::vector<entt::entity> scrap)
+        : scrap(std::move(scrap)) {}
+    const std::vector<entt::entity> scrap;
+};