mark trade ships plan-driven on spawn

Add `Game.markUnitPlanDriven()` so executions can suppress per-step Unit updates
before the first motion plan is recorded.

Use it in TradeShipExecution to drop the spawn-time placeholder motion plan, and
align TransportShipExecution re-plan `startTick` with `ticksPerMove`.

src/core/execution/TradeShipExecution.ts

@@ -8,7 +8,6 @@ import {
   UnitType,
 } from "../game/Game";
 import { TileRef } from "../game/GameMap";
-import { MotionPlanRecord } from "../game/MotionPlans";
 import { PathFinding } from "../pathfinding/PathFinder";
 import { PathStatus, SteppingPathFinder } from "../pathfinding/types";
 import { distSortUnit } from "../Util";
@@ -35,7 +34,6 @@ export class TradeShipExecution implements Execution {
   }
 
   tick(ticks: number): void {
-    let spawnedThisTick = false;
     if (this.tradeShip === undefined) {
       const spawn = this.origOwner.canBuild(
         UnitType.TradeShip,
@@ -50,19 +48,10 @@ export class TradeShipExecution implements Execution {
         targetUnit: this._dstPort,
         lastSetSafeFromPirates: ticks,
       });
+      // This unit can move immediately, but plan-driven units don't emit per-step Unit updates.
+      // Mark it plan-driven up-front so its first move doesn't generate redundant traffic.
+      this.mg.markUnitPlanDriven(this.tradeShip.id());
       this.mg.stats().boatSendTrade(this.origOwner, this._dstPort.owner());
-      spawnedThisTick = true;
-
-      const placeholderPlan: MotionPlanRecord = {
-        kind: "grid",
-        unitId: this.tradeShip.id(),
-        planId: this.motionPlanId,
-        startTick: ticks + 1,
-        ticksPerStep: 1,
-        path: [spawn],
-      };
-      this.mg.recordMotionPlan(placeholderPlan);
-      this.motionPlanDst = this._dstPort.tile();
     }
 
     if (!this.tradeShip.isActive()) {
@@ -146,7 +135,7 @@ export class TradeShipExecution implements Execution {
     }
 
     const dst = this._dstPort.tile();
-    if (spawnedThisTick || dst !== this.motionPlanDst) {
+    if (dst !== this.motionPlanDst) {
       this.motionPlanId++;
       const from = this.tradeShip.tile();
       const path = this.pathFinder.findPath(from, dst) ?? [from];

src/core/execution/TransportShipExecution.ts

@@ -282,7 +282,7 @@ export class TransportShipExecution implements Execution {
         kind: "grid",
         unitId: this.boat.id(),
         planId: this.motionPlanId,
-        startTick: ticks + 1,
+        startTick: ticks + this.ticksPerMove,
         ticksPerStep: this.ticksPerMove,
         path: fullPath,
       });

src/core/game/Game.ts

@@ -768,6 +768,11 @@ export interface Game extends GameMap {
   inSpawnPhase(): boolean;
   executeNextTick(): GameUpdates;
   drainPackedTileUpdates(): Uint32Array;
+  /**
+   * Marks a unit as "plan-driven" so its per-tile `Unit` updates can be suppressed.
+   * The client is expected to advance the unit position via motion plans.
+   */
+  markUnitPlanDriven(unitId: number): void;
   recordMotionPlan(record: MotionPlanRecord): void;
   drainPackedMotionPlans(): Uint32Array | null;
   setWinner(winner: Player | Team, allPlayersStats: AllPlayersStats): void;

src/core/game/GameImpl.ts

@@ -433,6 +433,10 @@ export class GameImpl implements Game {
     return packed;
   }
 
+  markUnitPlanDriven(unitId: number): void {
+    this.planDrivenUnitIds.add(unitId);
+  }
+
   recordMotionPlan(record: MotionPlanRecord): void {
     switch (record.kind) {
       case "grid":