refactor: tighten useAbility and Scenario archetype lookup

Final-review-driven changes (Tasks 6/M1, I1 from the ledger):

- Drop the redundant abilities-list check in CombatEngine::useAbility.
  By construction (Scenario::__construct enforces the allowlist), any
  ability the unit 'knows' is also archetype-allowed; the second check
  alone is sufficient and removes a dead code path.
- Update testUseAbilityRejectsAnAbilityTheArchetypeForbids to assert the
  'Unit archetype forbids that ability.' message it actually exercises.
- Compute $template once in Scenario::__construct's per-unit loop and
  drop the redundant catalog lookup on the next line.

tests/Unit/Domain/CombatEngineTest.php

@@ -971,7 +971,7 @@ final class CombatEngineTest extends TestCase
         );
 
         $this->expectException(CombatException::class);
-        $this->expectExceptionMessage('Unit does not know that ability.');
+        $this->expectExceptionMessage('Unit archetype forbids that ability.');
 
         (new CombatEngine())->useAbility($match, 'scout-1', 'heal', new Position(0, 1));
     }