feat(scene): built-in component codecs and UUID reference hooks #717

Summary

Register production YAML/binary codecs and UUID-reference hooks for every
currently authored built-in ECS component. The existing reflection registrations
are not serializer registrations, and static initialization is not an accepted
production lifecycle.

Scope

• Add explicit built-in scene-schema registration after feat(scene): stable component schema registry #714 has been initialized
  during engine startup.
• Implement codecs for the current document components: TransformComponent,
  ParentComponent, MeshComponent, CameraComponent, LightComponent,
  MaterialComponent, NameComponent, and RigidBodyComponent.
• Materialize the mandatory UUIDComponent from the owning entity record UUID;
  it is not a second component-map payload with an independently editable UUID.
• Classify authored, runtime-derived, editor-only, and forbidden fields. Runtime
  fields such as RenderInstanceId and GPU handles are excluded.
• Validate finite transforms, legal component values, UUID syntax, asset UUID
  references, and component-specific constraints.
• Implement stable field keys and UUID reference enumerate/patch hooks consumed
  by staged load, duplication, undo/redo, and Play restoration.
• Treat UUID component data as durable identity, enforce uniqueness through the
  scene-document path, and keep EntityID only in loader/runtime maps.

Non-goals

• Do not use static C++ registration, std::function, reflection offsets, or
  raw-byte serialization.
• Do not introduce a separate parent/hierarchy format outside the scene document
  and staged resolver.

Dependencies

• feat(scene): stable component schema registry #714 — stable component schema registry

Acceptance criteria

• All built-in authored values round-trip through stable YAML and binary codecs.
• Every loaded entity gets one UUIDComponent whose value exactly equals its
  SceneEntityRecord::UUID.
• Runtime-derived/render handles are absent after serialization and rebuilt by
  feat(scene): transactional scene load and runtime reconstruction #829.
• Invalid component payloads produce structured errors rather than being dropped.
• Tests cover codec values, field keys, reference enumeration, and rejection
  cases for each built-in component.

References

ZEngine/docs/future-plan/scene-serialization.md sections 2, 4, 6, and 11.

Implementation API contract

This function is the sole built-in registration entry point and is called by
engine startup immediately after the registry is initialized. It registers the
following fixed durable schema keys: zengine.ecs.transform,
zengine.ecs.parent, zengine.ecs.mesh, zengine.ecs.camera,
zengine.ecs.light, zengine.ecs.material, zengine.ecs.name, and
zengine.ecs.rigid_body. These names are source compatibility commitments, not
C++ type names.

namespace ZEngine::ECS
{
    [[nodiscard]] Core::VFS::VFSResult<void>
        RegisterBuiltInSceneSchemas(SceneComponentSchemaRegistry* registry,
                                    SceneDiagnostics* diagnostics);
}

Each schema supplies the SceneComponentCodecFns and SceneReferenceFns
defined by #714. ParentComponent uses a parent_uuid source field and is
resolved only after every candidate entity has been created. Mesh/material UUIDs
are enumerated as asset references. Duplication, clipboard, undo/redo, and Play
snapshot remapping operate on SceneValue payloads through Remap; they do not
patch a live ECS scene.

The required authored-field matrix is:

The loader—not a schema callback—adds UUIDComponent{SceneEntityRecord::UUID}
and rejects a conflicting component-map UUID entry. This prevents two durable
identity sources from drifting apart.