core(game): derive plan-driven state from motion plans

- Remove `markUnitPlanDriven` from `Game`/`GameImpl`
- Centralize “maybe emit UnitUpdate” behind `GameImpl.onUnitMoved`
- Record trade ship motion plan before the first move to avoid redundant per-step unit updates

src/core/execution/TradeShipExecution.ts

@@ -48,9 +48,6 @@ 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());
     }
 
@@ -109,12 +106,49 @@ export class TradeShipExecution implements Execution {
       return;
     }
 
-    const result = this.pathFinder.next(curTile, this._dstPort.tile());
+    const dst = this._dstPort.tile();
+    const result = this.pathFinder.next(curTile, dst);
 
     switch (result.status) {
       case PathStatus.PENDING:
+        if (dst !== this.motionPlanDst) {
+          this.motionPlanId++;
+          const from = curTile;
+          const path = this.pathFinder.findPath(from, dst) ?? [from];
+          if (path.length === 0 || path[0] !== from) {
+            path.unshift(from);
+          }
+
+          this.mg.recordMotionPlan({
+            kind: "grid",
+            unitId: this.tradeShip.id(),
+            planId: this.motionPlanId,
+            startTick: ticks + 1,
+            ticksPerStep: 1,
+            path,
+          });
+          this.motionPlanDst = dst;
+        }
         break;
       case PathStatus.NEXT:
+        if (dst !== this.motionPlanDst) {
+          this.motionPlanId++;
+          const from = result.node;
+          const path = this.pathFinder.findPath(from, dst) ?? [from];
+          if (path.length === 0 || path[0] !== from) {
+            path.unshift(from);
+          }
+
+          this.mg.recordMotionPlan({
+            kind: "grid",
+            unitId: this.tradeShip.id(),
+            planId: this.motionPlanId,
+            startTick: ticks + 1,
+            ticksPerStep: 1,
+            path,
+          });
+          this.motionPlanDst = dst;
+        }
         // Update safeFromPirates status
         if (this.mg.isWater(result.node) && this.mg.isShoreline(result.node)) {
           this.tradeShip.setSafeFromPirates();
@@ -133,26 +167,6 @@ export class TradeShipExecution implements Execution {
         this.active = false;
         return;
     }
-
-    const dst = this._dstPort.tile();
-    if (dst !== this.motionPlanDst) {
-      this.motionPlanId++;
-      const from = this.tradeShip.tile();
-      const path = this.pathFinder.findPath(from, dst) ?? [from];
-      if (path.length === 0 || path[0] !== from) {
-        path.unshift(from);
-      }
-
-      this.mg.recordMotionPlan({
-        kind: "grid",
-        unitId: this.tradeShip.id(),
-        planId: this.motionPlanId,
-        startTick: ticks + 1,
-        ticksPerStep: 1,
-        path,
-      });
-      this.motionPlanDst = dst;
-    }
   }
 
   private complete() {

src/core/game/Game.ts

@@ -778,11 +778,6 @@ 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

@@ -447,10 +447,6 @@ export class GameImpl implements Game {
     return packed;
   }
 
-  markUnitPlanDriven(unitId: number): void {
-    this.planDrivenUnitIds.add(unitId);
-  }
-
   recordMotionPlan(record: MotionPlanRecord): void {
     switch (record.kind) {
       case "grid":
@@ -466,10 +462,21 @@ export class GameImpl implements Game {
     this.motionPlanRecords.push(record);
   }
 
-  isUnitPlanDriven(unitId: number): boolean {
+  private isUnitPlanDriven(unitId: number): boolean {
     return this.planDrivenUnitIds.has(unitId);
   }
 
+  maybeAddUnitUpdate(unit: Unit): void {
+    if (!this.isUnitPlanDriven(unit.id())) {
+      this.addUpdate(unit.toUpdate());
+    }
+  }
+
+  onUnitMoved(unit: Unit): void {
+    this.updateUnitTile(unit);
+    this.maybeAddUnitUpdate(unit);
+  }
+
   drainPackedMotionPlans(): Uint32Array | null {
     const records = this.motionPlanRecords;
     if (records.length === 0) {

src/core/game/UnitImpl.ts

@@ -159,10 +159,7 @@ export class UnitImpl implements Unit {
     }
     this._lastTile = this._tile;
     this._tile = tile;
-    this.mg.updateUnitTile(this);
-    if (!this.mg.isUnitPlanDriven(this._id)) {
-      this.mg.addUpdate(this.toUpdate());
-    }
+    this.mg.onUnitMoved(this);
   }
 
   setTroops(troops: number): void {