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

tests/core/executions/TradeShipExecution.test.ts

@@ -74,9 +74,11 @@ describe("TradeShipExecution", () => {
     tradeShip = {
       isActive: vi.fn(() => true),
       owner: vi.fn(() => origOwner),
+      id: vi.fn(() => 123),
       move: vi.fn(),
       setTargetUnit: vi.fn(),
       setSafeFromPirates: vi.fn(),
+      touch: vi.fn(),
       delete: vi.fn(),
       tile: vi.fn(() => 2001),
     } as any;
@@ -85,6 +87,7 @@ describe("TradeShipExecution", () => {
     tradeShipExecution.init(game, 0);
     tradeShipExecution["pathFinder"] = {
       next: vi.fn(() => ({ status: PathStatus.NEXT, node: 2001 })),
+      findPath: vi.fn((from: number) => [from]),
     } as any;
     tradeShipExecution["tradeShip"] = tradeShip;
   });
@@ -116,6 +119,7 @@ describe("TradeShipExecution", () => {
   it("should complete trade and award gold", () => {
     tradeShipExecution["pathFinder"] = {
       next: vi.fn(() => ({ status: PathStatus.COMPLETE, node: 2001 })),
+      findPath: vi.fn((from: number) => [from]),
     } as any;
     tradeShipExecution.tick(1);
     expect(tradeShip.delete).toHaveBeenCalledWith(false);