fix: enforce executable movement state

src/Domain/CombatEngine.php

@@ -4,13 +4,19 @@ declare(strict_types=1);
 
 namespace BattleForge\Domain;
 
+use InvalidArgumentException;
+
 final class CombatEngine
 {
     public function move(MatchState $match, string $unitId, Position $destination): MatchState
     {
         $this->assertMatchActive($match);
 
-        $unit = $match->unit($unitId);
+        try {
+            $unit = $match->unit($unitId);
+        } catch (InvalidArgumentException $exception) {
+            throw new CombatException("Unknown unit: {$unitId}.", previous: $exception);
+        }
         $this->assertCanAct($match, $unit);
 
         $occupied = [];

src/Domain/MatchState.php

@@ -30,6 +30,7 @@ final readonly class MatchState
 
         $unitIds = [];
         $teamIds = [];
+        $occupiedPositions = [];
 
         foreach ($units as $unit) {
             if (!self::isUnitState($unit)) {
@@ -40,6 +41,24 @@ final readonly class MatchState
                 throw new InvalidArgumentException("Duplicate unit id: {$unit->id}.");
             }
 
+            if (!$battlefield->contains($unit->position)) {
+                throw new InvalidArgumentException("Unit {$unit->id} is outside the battlefield.");
+            }
+
+            if (!$unit->isDefeated() && $battlefield->terrainAt($unit->position)->movementCost() === null) {
+                throw new InvalidArgumentException("Living unit {$unit->id} must stand on passable terrain.");
+            }
+
+            $positionKey = $unit->position->key();
+
+            if (!$unit->isDefeated() && isset($occupiedPositions[$positionKey])) {
+                throw new InvalidArgumentException("Living units cannot share position {$positionKey}.");
+            }
+
+            if (!$unit->isDefeated()) {
+                $occupiedPositions[$positionKey] = true;
+            }
+
             $unitIds[$unit->id] = true;
             $teamIds[$unit->teamId] = true;
         }