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/client/graphics/layers/UnitLayer.ts

@@ -65,11 +65,27 @@ export class UnitLayer implements Layer {
   }
 
   tick() {
-    const unitIds = this.game
-      .updatesSinceLastTick()
-      ?.[GameUpdateType.Unit]?.map((unit) => unit.id);
+    const updatedUnitIds =
+      this.game
+        .updatesSinceLastTick()
+        ?.[GameUpdateType.Unit]?.map((unit) => unit.id) ?? [];
 
-    this.updateUnitsSprites(unitIds ?? []);
+    const motionPlanUnitIds = this.game.motionPlannedUnitIds();
+
+    if (updatedUnitIds.length === 0) {
+      this.updateUnitsSprites(motionPlanUnitIds);
+      return;
+    }
+    if (motionPlanUnitIds.length === 0) {
+      this.updateUnitsSprites(updatedUnitIds);
+      return;
+    }
+
+    const unitIds = new Set<number>(updatedUnitIds);
+    for (const id of motionPlanUnitIds) {
+      unitIds.add(id);
+    }
+    this.updateUnitsSprites(Array.from(unitIds));
   }
 
   init() {