rendering: integrate DrawList with SoRenderManager

[tritao]

Summary

Rebuild the manager/orchestration layer on the retained render-plan and picking foundation.

This layer owns:

• DrawList pipeline selection and backend creation;
• camera, viewport, and device-pixel-ratio forwarding;
• manager callbacks and render-root orchestration;
• deterministic backend replacement while the current context is valid.

Selection meaning and lifetime are deliberately not manager responsibilities. Coin receives frame-local SoSelectionState targets produced during traversal and later executes them through the stage-aware backend plan. FreeCAD and other applications retain their own selection/highlight policy and identity model.

Stack

This layer is built on the private render-plan boundary and transient picking/selection mechanics from PR #52.