The v1.2 RawProperty path already deep-parses ArrayProperty<ObjectProperty>
(see PropertySerializer.cs:240) but falls through to the binary-size fence
for ArrayProperty<StructProperty>, leaving callers with unparseable bytes.
That's the gap blocking ERP.Satisfactory #138: pipe actors carry
`mSplineData` as Array<Struct<FSplinePointData>>, and the app can't render
pipe polylines without those inner property lists.

Wire format at v1.2 (inferred from the existing Array<Object> branch and
Unreal property conventions): the inner-tag header that v1.1 carried
before the element loop is gone — the struct subtype name lives in
TypeNode.Children[0] (e.g. "SplinePointData"). Value bytes are int32
count, then `count` element bodies, each a v1.2 property-list terminated
by "None". No per-element serializationControl byte (those only prefix
the outer object body).

Changes
- RawProperty: new `ArrayStructValues` slot exposing `StructElementValue`
  entries (each carries the inner Property list).
- PropertySerializer: new `IsStructChild` predicate + Array<Struct> branch
  in TryParseKnownValue. Reads count, drains v1.2 property-lists per
  element, respects the outer binarySize fence to recover if the format
  guess is off (ArrayStructValues stays null on misread).
- Threaded `Header` into TryParseKnownValue so the inner DeserializeProperty
  loop can resolve struct sub-property reads that need header context.

Tests
- PropertySerializerV12Tests:
  ArrayOfStruct_ParsesElementsAsInnerPropertyLists — two elements, each
  one IntProperty + "None" terminator; verifies inner values and that
  the outer fence aligns to the post-value sentinel.
- ArrayOfStruct_EmptyArray_ProducesEmptyValuesList — count=0 fast path.
- All 137 existing tests continue to pass.

Downstream: unblocks ERP.Satisfactory #138 (pipe polylines via
Pipeline.Polyline) and lands the app on 0.4.0.