Fix Owner reference-cycle handling across all serializers

[fuffc]

Fix Owner reference-cycle handling across all serializers

Factory.InitLayout wires each dockable's Owner back-reference, producing a cyclic object graph (Dockable.Owner → Dock → VisibleDockables → Dockable). Saving and reloading such a layout was broken in four of the five serializers — ranging from silent identity duplication to process-killing crashes. This PR fixes each one and adds round-trip regression coverage.

What was broken

• System.Text.Json — the per-item JsonConverterList restarted STJ's reference tracking on every element, breaking $id/$ref across list boundaries and duplicating ActiveDockable identity.
• Xml / Yaml — ListTypeConverter's post-deserialize tree walk had no cycle guard, so once the Owner back-reference was restored it recursed forever — an uncatchable StackOverflowException that kills the process.
• Protobuf — protobuf-net 3.x has no working reference preservation (AsReference is obsolete-as-error), so the cycle either threw or came back with duplicate instances. A separate "Unexpected sub-type" failure also broke any custom Dockable subclass defined outside the Dock.Model.* assemblies.

Changes

1. Fix System.Text.Json Owner reference-cycle handling — replace JsonConverterList/JsonConverterFactoryList with DockListTypeInfoModifier, a JsonTypeInfo resolver modifier that only overrides list instantiation, leaving STJ's built-in enumerable path (and its single ambient reference-tracking state) in control of the whole graph.
2. Fix Xml/Yaml StackOverflowException on Owner reference cycles — add a reference-equality visited set to ListTypeConverter; the walk is idempotent, so visiting each instance once is sufficient.
3. Fix Protobuf Owner cycle crash and ActiveDockable duplication — exclude Owner/OriginalOwner from the wire; ListTypeConverter rebuilds Owner structurally from tree containment (no InitLayout call required, matching the other serializers) and reconciles ActiveDockable/DefaultDockable/FocusedDockable by Id, guarding against null/duplicate Ids.
4. Fix Protobuf Unexpected sub-type for custom Dockable subclasses — register each class under its nearest tracked base class as well as its closest interface.
5. Preserve Dock list constructors for NativeAOT — statically root the built-in IList<IDockable> and IList<IDockWindow> creators so trimmed source-generated applications retain the required constructors.

Testing

• DockLayoutReferencePreservationTests round-trips a wired layout through all five serializers and asserts ActiveDockable identity is preserved.
• Protobuf-specific tests cover structural Owner rebuild and custom-subtype round-tripping.
• System.Text.Json tests cover AOT-safe observable Dock collections.
• GitHub Actions: all 9 checks pass, including the NativeAOT source-generation sample and packaging.

Closes #1104
Closes #1105