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.

src/Domain/Scenario.php

@@ -72,12 +72,11 @@ final readonly class Scenario
                 throw new InvalidArgumentException("Unit {$unit->id} is outside the battlefield.");
             }
 
-            if (!ArchetypeCatalog::templates()[$unit->archetype->value] ?? false) { // @phpstan-ignore nullCoalesce.expr, if.alwaysFalse, booleanNot.alwaysFalse (Runtime guard: defense in depth against an unknown Archetype enum value.)
+            $template = ArchetypeCatalog::templates()[$unit->archetype->value] ?? null;
+            if ($template === null) {
                 throw new InvalidArgumentException("Unit {$unit->id} uses an unknown archetype.");
             }
 
-            $template = ArchetypeCatalog::templates()[$unit->archetype->value];
-
             if ($unit->maxHealth < $template->minHealth || $unit->maxHealth > $template->maxHealth) {
                 throw new InvalidArgumentException("Unit {$unit->id} health is outside archetype bounds.");
             }