delete Pixi build-ghost rendering from StructureIconsLayer

The WebGL ghost (StructurePass + RangeCirclePass + RailroadPass +
CrosshairPass) is fed via GhostPreviewUpdatedEvent and looks correct;
the Pixi-rendered ghost was a duplicate.

Strip the Pixi side out of StructureIconsLayer:
- delete imports for pixi.js / pixi-filters / colord plugins / Theme /
  SpriteFactory / StructureDrawingUtils / bitmapFont / renderNumber
- delete fields: pixicanvas, ghostStage, rootStage, renderer,
  rendererInitialized, theme, factory, filterRedArray, rebuildPending,
  and the Pixi half of ghostUnit (container, priceText, priceBg,
  priceGroup, priceBox, range, rangeLevel, targetingAlly) — now just
  { buildableUnit }
- delete methods: setupRenderer, redraw, rendererOrGLContextLost,
  resizeCanvas, renderLayer, shouldTransform, updateGhostPrice,
  updateGhostRange, plus the Pixi guts of moveGhost /
  createGhostStructure / clearGhostStructure

init() is now sync; the per-RAF state checks move to a tick()-driven
syncGhostState() + renderGhost() (renderGhost still re-queries
buildables, just no longer paints anything).

Net: ~250 LOC gone, no canvas2D drawing left in the layer. The
canvas2D map canvas itself has no remaining writers — ready to be
deleted in a follow-up.

src/client/InputHandler.ts

@@ -96,6 +96,17 @@ export class GhostStructureChangedEvent implements GameEvent {
   constructor(public readonly ghostStructure: PlayerBuildableUnitType | null) {}
 }
 
+/**
+ * Per-frame ghost preview state for the WebGL renderer. Emitted by the
+ * canvas2D ghost layer; consumed in ClientGameRunner.mountWebGLDebugRenderer
+ * to push to view.updateGhostPreview. `data` is null when no ghost is active.
+ */
+export class GhostPreviewUpdatedEvent implements GameEvent {
+  constructor(
+    public readonly data: import("./render/types").GhostPreviewData | null,
+  ) {}
+}
+
 export class ConfirmGhostStructureEvent implements GameEvent {}
 
 export class SwapRocketDirectionEvent implements GameEvent {