rendering: integrate Coin render layers with FreeCAD

[tritao]

Summary

Add the render-layer and multi-stage orchestration needed by FreeCAD on top of the retained DrawList manager integration.

• Add explicit `Background`, `Main`, `AfterMain`, and `Foreground` render stages.
• Add `SoRenderLayerGroup` for semantic subtree placement and local rendering behavior.
• Support layer-local viewport overrides consistently in LegacyGL and DrawList traversal.
• Support scoped depth clearing at the correct layer boundary and effective viewport.
• Add manager-owned background and foreground roots with sensors and invalidation.
• Add after-main traversal callbacks while preserving deterministic stage ordering.
• Apply manager lighting policy consistently across all render stages.
• Expose the concrete manager hooks required by FreeCAD without introducing a generalized render graph or pass scheduler.
• Support orderly backend resource release while a GL context is valid.
• Support no-GL resource discard after context loss and clean reinitialization on a new context.
• Preserve picking resources correctly across both release/reinitialize and discard/context-replacement paths.
• Add focused native coverage for stage ordering, foreground placement, viewport behavior, scoped depth clearing, invalidation, lighting, context recovery, and picking after resource recreation.

The retained IR remains backend-neutral and the OpenGL backend remains responsible only for executing retained commands and managing GPU resources. LegacyGL provides compatibility behavior without defining the retained architecture.

Stack

Layer 16/16, based on PR #33.

[tritao]

Superseded by the rebuilt layer stack in #54 and #55, based on the rewritten picking layer #52.