When assigning into a nested array dim fetch like `$ret[$val]['cat2'] = 'blah2'`
with a union offset `$val`, the outer `$ret` gets merged via
`setOffsetValueType()` which widens value types with a union. The directly
tracked expression `$ret[$val]` in the Scope holds more precise information,
but it was being re-derived from the widened outer `$ret` via
`getOffsetValueType()` when computing additional expressions and when
drilling down for subsequent levels. This caused previously-set keys to
become optional again on the next chained assignment.

- When drilling into nested levels in `produceArrayDimFetchAssignValueToWrite`,
  prefer `Scope::getType()` for intermediate dim fetches that are tracked in
  Scope, instead of re-deriving from the parent array.
- When producing `additionalExpressions`, use the intermediate value computed
  during the reverse assignment loop instead of drilling into the widened
  outer container.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

Closes phpstan/phpstan#13304
Closes phpstan/phpstan#13643

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>