validate coords in construction execution (#1339)

## Description:

Passing in invalidate coordinates in a BuildIntent can cause
ExecutionManager to throw an exception because the coords are not
validated.

## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced
- [x] I understand that submitting code with bugs that could have been
caught through manual testing blocks releases and new features for all
contributors

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

evan

src/core/execution/ExecutionManager.ts

@@ -1,4 +1,4 @@
-import { Execution, Game } from "../game/Game";
+import { Cell, Execution, Game } from "../game/Game";
 import { PseudoRandom } from "../PseudoRandom";
 import { ClientID, GameID, Intent, Turn } from "../Schemas";
 import { simpleHash } from "../Util";
@@ -107,11 +107,10 @@ export class Executor {
       case "embargo":
         return new EmbargoExecution(player, intent.targetID, intent.action);
       case "build_unit":
-        // TODO: fix this
         return new ConstructionExecution(
           player,
-          this.mg.ref(intent.x, intent.y),
           intent.unit,
+          new Cell(intent.x, intent.y),
         );
       case "allianceExtension": {
         return new AllianceExtensionExecution(player, intent.recipient);