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/TransportShipExecution.ts

@@ -277,7 +277,7 @@ 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,
       });