Add comment explaining why instanceof UnionType is the correct guard

The `instanceof UnionType` check in filterBySpecifiedTypes is a structural
check (is this type an explicit union of alternatives?), not a semantic
type check (is this a string/int/etc?). It's the correct restriction because:

- Union types represent disjunctions: narrowing to one member validly
  satisfies the guard condition (e.g., 1|2 narrowed to 1)
- Non-union types like IntersectionType can have coincidental subtype
  relationships that cause incorrect type narrowing when the conditional
  result is applied (e.g., non-falsy-string with 'filter' as subtype)
- BooleanType (which represents true|false but is not a UnionType) must be
  excluded because template conditional types create per-branch conditionals
  that would conflict via subtype matching

Alternative approaches explored and rejected:
- TypeCombinator::remove-based check: correctly handles unions and
  intersections, but incorrectly allows BooleanType guards (causing
  *NEVER* type in template conditional assertions like bug-14249)
- !instanceof MixedType: too permissive, allows intersection type guards
- getFiniteTypes() > 1: too restrictive, excludes non-finite unions
  like int|string

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: phpstan-bot

src/Analyser/MutatingScope.php

@@ -3227,6 +3227,13 @@ public function filterBySpecifiedTypes(SpecifiedTypes $specifiedTypes): self
 							continue;
 						}
 						$guardType = $conditionalTypeHolder->getType();
+						// Allow subtype matching: when the specified type is a strict subtype
+						// of the guard, fire the conditional. Restricted to UnionType guards
+						// because unions represent explicit disjunctions of alternatives —
+						// narrowing to one alternative validly satisfies the guard condition.
+						// Non-union types (IntersectionType, BooleanType, etc.) can have
+						// coincidental subtype relationships that cause incorrect type
+						// narrowing when the conditional result is applied to the scope.
 						if (
 							$conditionalExpression->getTypeHolder()->getCertainty()->yes()
 							&& $specifiedHolder->getCertainty()->yes()