Write a shape's optional keys as optional into a general array's item shape

ArrayType::setExistingOffsetValueType() merged a written constant array
into the item shape all-or-nothing: once with every key required and once
with every optional key unset. A written shape whose optional keys can be
present independently (a nested `$a[$i][$j][$k]['x'] = ...` write in a loop)
then came out as `array{}|array{k1: T, ..., kN: T}`, which is unsound for
a value holding only some of the keys. Optional keys are now written as
optional: an existing key keeps its certainty and unions the value, a
missing one is added as optional.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DfQHLTyyoMfjfa4sXJCGa5

Author: ondrejmirtes

src/Type/ArrayType.php

@@ -428,7 +428,22 @@ public function setExistingOffsetValueType(Type $offsetType, Type $valueType): T
 		if ($this->itemType->isConstantArray()->yes() && $valueType->isConstantArray()->yes()) {
 			$newItemTypes = [];
 
+			$itemConstantArrays = $this->itemType->getConstantArrays();
 			foreach ($valueType->getConstantArrays() as $constArray) {
+				if ($constArray->getOptionalKeys() !== [] && count($itemConstantArrays) === 1) {
+					// A written shape with optional keys is not all-or-nothing: each
+					// optional key may or may not be present on its own, so it is
+					// written as optional (present keys keep their certainty, the
+					// value unions with what the key held) instead of once with every
+					// key required and once with every optional key unset.
+					$builder = ConstantArrayTypeBuilder::createFromConstantArray($itemConstantArrays[0]);
+					foreach ($constArray->getKeyTypes() as $i => $keyType) {
+						$builder->setOffsetValueType($keyType, $constArray->getOffsetValueType($keyType), $constArray->isOptionalKey($i));
+					}
+					$newItemTypes[] = TypeCombinator::intersect($builder->getArray(), ...TypeUtils::getAccessoryTypes($this->itemType));
+					continue;
+				}
+
 				$newItemType = $this->itemType;
 				$optionalKeyTypes = [];
 				foreach ($constArray->getKeyTypes() as $i => $keyType) {