Packed unit updates / MotionPlans (#3292)

## Description:

Reduce per-step `Unit` update traffic by shipping packed motion plans
and letting the client advance plan-driven units locally.

Changes:
- Add packed motion plan records (`packedMotionPlans?: Uint32Array`) to
game updates and transfer the buffer worker -> main.
- Introduce `src/core/game/MotionPlans.ts` (schema + pack/unpack) for
grid + train motion plans.
- Extend `Game` with `recordMotionPlan(...)` and
`drainPackedMotionPlans()`, and implement buffering/packing in
`GameImpl`.
- Treat units with motion plans as “plan-driven”: suppress per-tile
`Unit` updates on `move()` and advance positions client-side.
- Emit motion plans from executions:
- `TradeShipExecution`: record/update grid motion plans and `touch()`
when changing target after capture.
- `TransportShipExecution`: record initial plan and update it when
destination changes.
  - `TrainExecution`: record a train plan on init (engine + cars).
- Client: apply motion plans in `GameView` and ensure `UnitLayer`
updates sprites for motion-planned units even when no `Unit` updates
arrived.

## Please complete the following:

- [ ] I have added screenshots for all UI updates
- [ ] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [ ] I have added relevant tests to the test directory
- [ ] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced

## Please put your Discord username so you can be contacted if a bug or
regression is found:

DISCORD_USERNAME

src/core/game/UnitImpl.ts

@@ -159,8 +159,7 @@ export class UnitImpl implements Unit {
     }
     this._lastTile = this._tile;
     this._tile = tile;
-    this.mg.updateUnitTile(this);
-    this.mg.addUpdate(this.toUpdate());
+    this.mg.onUnitMoved(this);
   }
 
   setTroops(troops: number): void {
@@ -336,7 +335,10 @@ export class UnitImpl implements Unit {
     if (this.type() !== UnitType.TransportShip) {
       throw new Error(`Cannot retreat ${this.type()}`);
     }
-    this._retreating = true;
+    if (!this._retreating) {
+      this._retreating = true;
+      this.mg.addUpdate(this.toUpdate());
+    }
   }
 
   isUnderConstruction(): boolean {