move multi-unit warship selection box to WebGL SelectionBoxPass

SelectionBoxPass now stores an array of selections and renders one
quad per entry. GPURenderer gains setSelectedUnits(ids) — the
single-unit setSelectedUnit becomes a wrapper. Position + color are
rebuilt each frame from lastUnits; dead unit IDs get pruned in place.

ClientGameRunner's UnitSelectionEvent listener forwards both single
and multi to view.setSelectedUnits — no more single/multi split.

UILayer drops everything canvas2D-related: the offscreen canvas +
context, theme, selectionAnimTime, multiSelectionBoxCenters,
SELECTION_BOX_SIZE, drawSelectionBoxMulti, paintSelectionBoxAt,
clearSelectionBox, paintCell, clearCell, and renderLayer / redraw /
shouldTransform. tick() now only prunes destroyed warships from the
selection list; the layer is purely state + click handling. ~120 LOC
gone.

Tests: UILayer.test.ts updated — drops the canvas/redraw asserts,
adds a multi-selection state assertion.

tests/client/graphics/UILayer.test.ts

@@ -28,17 +28,8 @@ describe("UILayer", () => {
     transformHandler = {};
   });
 
-  it("should initialize and redraw canvas", () => {
-    const ui = new UILayer(game, eventBus, transformHandler);
-    ui.redraw();
-    expect(ui["canvas"].width).toBe(100);
-    expect(ui["canvas"].height).toBe(100);
-    expect(ui["context"]).not.toBeNull();
-  });
-
   it("tracks the selected unit on single-unit selection (rendering is WebGL)", () => {
     const ui = new UILayer(game, eventBus, transformHandler);
-    ui.redraw();
     const unit = {
       type: () => "Warship",
       isActive: () => true,
@@ -48,14 +39,13 @@ describe("UILayer", () => {
     const event = { isSelected: true, unit };
     ui["onUnitSelection"](event as UnitSelectionEvent);
     // selectedUnit is held for game-logic callers (the click handlers). The
-    // visual selection box is now drawn by WebGL SelectionBoxPass — wired
-    // from ClientGameRunner via view.setSelectedUnit(unit.id()).
+    // visual selection box is drawn by WebGL SelectionBoxPass — wired from
+    // ClientGameRunner via view.setSelectedUnits([unit.id()]).
     expect(ui["selectedUnit"]).toBe(unit);
   });
 
   it("clears selection on deselect", () => {
     const ui = new UILayer(game, eventBus, transformHandler);
-    ui.redraw();
     const unit = {
       type: () => "Warship",
       isActive: () => true,
@@ -69,4 +59,19 @@ describe("UILayer", () => {
     } as unknown as UnitSelectionEvent);
     expect(ui["selectedUnit"]).toBeNull();
   });
+
+  it("tracks multi-selection list", () => {
+    const ui = new UILayer(game, eventBus, transformHandler);
+    const units = [
+      { id: () => 1, isActive: () => true },
+      { id: () => 2, isActive: () => true },
+    ];
+    ui["onUnitSelection"]({
+      isSelected: true,
+      unit: null,
+      units,
+    } as unknown as UnitSelectionEvent);
+    expect(ui["multiSelectedWarships"]).toEqual(units);
+    expect(ui["selectedUnit"]).toBeNull();
+  });
 });