move single-unit warship selection box to WebGL SelectionBoxPass

UnitSelectionEvent now forwards to view.setSelectedUnit(unit.id()) in
mountWebGLDebugRenderer; the renderer's SelectionBoxPass draws the
animated stippled outline on the GPU. UILayer still tracks
selectedUnit for game-logic readers (the click handlers) but no longer
paints to canvas2D for it.

Drops drawSelectionBox + lastSelectionBoxCenter (~50 LOC) plus the
per-tick single-unit redraw in tick(). Multi-selection stays on
canvas2D — SelectionBoxPass is single-unit only.

Test update: replaces the now-dead drawSelectionBox spy with a
selectedUnit state assertion + a deselect case.

src/client/ClientGameRunner.ts

@@ -43,6 +43,7 @@ import {
   MouseMoveEvent,
   MouseUpEvent,
   TickMetricsEvent,
+  UnitSelectionEvent,
 } from "./InputHandler";
 import { endGame, startGame, startTime } from "./LocalPersistantStats";
 import { terrainMapFileLoader } from "./TerrainMapFileLoader";
@@ -345,6 +346,23 @@ function mountWebGLDebugRenderer(
     view.showMoveIndicator(tx, ty, firstUnit.owner().smallID());
   });
 
+  // Single-unit warship selection box: forward UnitSelectionEvent to the
+  // renderer's SelectionBoxPass. Multi-selection (event.units.length > 0)
+  // stays canvas2D for now — SelectionBoxPass only supports one unit.
+  eventBus.on(UnitSelectionEvent, (e) => {
+    if (!e.isSelected) {
+      view.setSelectedUnit(null);
+      return;
+    }
+    if ((e.units ?? []).length > 0) {
+      // Multi-selection: drop any prior single highlight; canvas2D draws
+      // the multi outlines in UILayer.
+      view.setSelectedUnit(null);
+      return;
+    }
+    view.setSelectedUnit(e.unit?.id() ?? null);
+  });
+
   return { builder: new WebGLFrameBuilder(view), syncCamera };
 }