rendering: add retained render layers and replay lifecycle

[tritao]

Summary

Rebuild the render-layer and replay/lifecycle layer on the frozen manager and planner stack.

This layer owns:

• background, main, after-main, and foreground stage membership;
• local viewport overrides and scoped depth clears;
• delayed traversal with captured replay context;
• valid-context shutdown versus lost-context discard;
• authoritative stage and depth-clear ordering in the retained render plan, executed by the OpenGL backend.

Selection overlays are supplied as frame-local SoSelectionState targets by traversal. They are queued and flushed at the plan's depth-segment barriers, so they execute in the same authoritative timing as their selected draw commands and before a subsequent depth clear can destroy that depth segment. Picking consumes the same retained render plan for draw and depth-clear ordering; ordinary visual rendering does not populate the pick target implicitly.

Stack

This layer is built on the manager/orchestration layer and completes plan-native selection timing, layer depth barriers, and retained lifecycle execution.