add FieldSelectionPanel for the ships/stations/debris selection

SelectedBuildingPanel has grown to 1200 lines by carrying two unrelated
selection categories. Introduce the field half as its own widget first,
so the cut-over is a separate, reviewable step.

The panel owns only its own selection state and widgets: it renders the
single-object stats panel (ship, station, debris) or the multi-object
count summary, subscribes to the tick/commands-applied refresh signals
and the debug-draw toggle, and hides itself while it has no selection.
Which category owns the side panel is not its decision - the parent
feeds it through setSelectedEntities/setSelectedDebris/clearSelection.

Co-Authored-By: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YHcUerKAZKWNvSKJxYKbnG

src/ui/FieldSelectionPanel.h

@@ -0,0 +1,91 @@
+#pragma once
+
+#include <vector>
+
+#include <QString>
+#include <QWidget>
+
+#include "entt/entity/entity.hpp"
+
+#include "DebugDrawToggledEvent.h"
+#include "EventHandler.h"
+#include "PlayerCommandsAppliedEvent.h"
+#include "TickAdvancedEvent.h"
+
+struct GameConfig;
+class Simulation;
+class ShipStatsPanel;
+class QLabel;
+class QVBoxLayout;
+
+// Renders the "field" selection category — ships, defence stations and debris — as either
+// a single-object stats panel (ship, station, or debris) or a compact multi-object count
+// summary (REQ-UI-SELECTION-CATEGORIES, REQ-UI-FIELD-MULTI-SELECTION, REQ-UI-DEBRIS-PANEL).
+//
+// The panel owns its own selection state and its own widgets, and nothing else. Which of
+// the two selection categories owns the side panel is arbitrated by the parent
+// SelectedBuildingPanel: it feeds this panel through setSelectedEntities() /
+// setSelectedDebris() / clearSelection() and asks it via hasSelection(). This panel hides
+// itself whenever its selection is empty, so an inactive field category takes no space.
+class FieldSelectionPanel : public QWidget,
+                            public CombinedEventHandler<TickAdvancedEvent,
+                                                        PlayerCommandsAppliedEvent,
+                                                        DebugDrawToggledEvent>
+{
+    Q_OBJECT
+
+public:
+    FieldSelectionPanel(Simulation* sim, const GameConfig* config,
+                        QWidget* parent = nullptr);
+    ~FieldSelectionPanel() override;
+
+    // Replaces the selected actors (ships and defence stations); debris is left alone,
+    // the two coexist within the field category (REQ-UI-SELECTION-CATEGORIES).
+    void setSelectedEntities(const std::vector<entt::entity>& entities);
+    // Replaces the selected debris; the selected actors are left alone.
+    void setSelectedDebris(const std::vector<entt::entity>& debris);
+    // Drops the whole field selection — used when the building category takes over.
+    void clearSelection();
+    // True while the field category has anything selected, i.e. while this panel owns
+    // the side panel's content.
+    bool hasSelection() const;
+
+private:
+    void handleEvent(std::shared_ptr<const TickAdvancedEvent> event) override;
+    void handleEvent(std::shared_ptr<const PlayerCommandsAppliedEvent> event) override;
+    void handleEvent(std::shared_ptr<const DebugDrawToggledEvent> event) override;
+
+    // Picks the layout for the current selection and shows/hides this panel accordingly.
+    void rebuild();
+    // Keeps the live values of the layout chosen by rebuild() current.
+    void refreshDisplay();
+    void buildEntityShip(entt::entity entity);
+    void buildEntityStation(entt::entity entity);
+    void buildEntitySummary();
+    void buildDebrisSingle();
+    void refreshEntityStats();
+    void hideAllWidgets();
+    // Summed remaining scrap across the selected debris (REQ-UI-DEBRIS-PANEL).
+    int selectedDebrisScrapTotal() const;
+    // "Scrap x N" line for the multi-object summary (REQ-UI-FIELD-MULTI-SELECTION).
+    QString scrapTotalText() const;
+
+    Simulation*       m_sim;
+    const GameConfig* m_config;
+
+    bool m_debugDraw = false;
+
+    // The selected ships/defence stations. Shares the "field" selection category with
+    // debris (m_selectedDebris): both can be non-empty at once (REQ-UI-SELECTION-CATEGORIES).
+    std::vector<entt::entity> m_selectedEntities;
+    std::vector<entt::entity> m_selectedDebris;
+
+    QVBoxLayout*    m_layout;
+    QLabel*         m_entityTitleLabel;
+    ShipStatsPanel* m_entityStatsPanel;
+    QLabel*         m_stationStatsLabel;
+    QLabel*         m_entitySummaryLabel;
+    // Shows the debris "Scrap" stat row (single selection) — the scrap total for the
+    // multi-object summary lives in m_entitySummaryLabel instead.
+    QLabel*         m_scrapLabel;
+};