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/ecs/system/ScrapSystem.cpp

@@ -69,9 +69,9 @@ std::vector<ScrapInfo> ScrapSystem::allScrapInfo() const
 {
     std::vector<ScrapInfo> result;
     m_admin.forEach<ScrapDataComponent>(
-        [&result, this](entt::entity e, const ScrapDataComponent& /*sd*/)
+        [&result, this](entt::entity e, const ScrapDataComponent& sd)
         {
-            result.push_back(ScrapInfo{e, m_admin.get<PositionComponent>(e).value});
+            result.push_back(ScrapInfo{e, m_admin.get<PositionComponent>(e).value, sd.amount});
         });
     return result;
 }