rendering: add retained render layers and replay lifecycle

[tritao]

Builds on #54 and completes the retained renderer's semantic and lifecycle
foundation before the performance work begins.

Purpose

Real scenes are not rendered as one uninterrupted list. They contain overlays,
ordered layers, local viewport or camera changes, and explicit depth clears.
Some nodes also delay work until traversal state would normally have gone out
of scope. The retained renderer therefore needs a single authoritative model
for ordering that work and for deciding which state survives until replay.

This PR introduces retained render layers and makes the render plan the source
of truth for visual drawing, selection, and picking. It also defines how
backend resources are released during normal shutdown and discarded after
context loss.

What changed

• Add SoRenderLayerGroup for stage-aware layer traversal and scoped depth
  clears.
• Capture the replay context needed by delayed traversal instead of relying on
  expired action state.
• Execute layer operations and selection at render-plan depth boundaries.
• Make picking follow the same visual depth segments as rendering.
• Distinguish normal backend shutdown from context-loss resource discard.
• Expose renderer capability, pipeline outcome, and matrix-policy contracts
  needed by manager and application integrations.

Ownership and scope

The planner owns execution order. The backend owns OpenGL resources. The
manager orchestrates the pipeline, while selection policy remains outside it.

This layer defines renderer semantics and lifecycle only. Benchmark tooling
and performance optimizations start in #67.

Validation

• Render-layer ordering, viewport overrides, and scoped depth clears.
• Delayed replay with preserved traversal context.
• Selection and picking across visual depth segments.
• Backend shutdown, context-loss discard, and resource recovery.
• Core-only and compatibility configurations, including Windows shared-library
  boundaries for non-exported implementation fixtures.