rendering: reuse unchanged retained frames

[tritao]

Builds on #67.

Purpose

The first retained-renderer path still traversed the scene graph, rebuilt the
draw list and render plan, and revalidated backend resources on every frame,
even when nothing relevant had changed. That made a steady viewport pay most
of the same CPU cost as a newly constructed frame.

This PR gives SoRenderManager an unchanged-frame reuse path. When the scene,
camera, viewport, and other retained inputs are still valid, the manager can
execute the existing draw list and plan instead of rebuilding them.

What changed

• Reuse an unchanged retained frame and its validated backend resources.
• Keep forced rebuilding available for correctness checks and direct
  before/after benchmark comparisons.
• Add opt-in timing for traversal, DrawList construction, planning, backend
  preparation and submission, GPU completion, selection, picking, and the
  total frame.
• Report phases separately in benchmark JSON so moving work between phases
  cannot look like a free optimization.

Safety and scope

Reuse is conditional on the retained inputs remaining valid; otherwise the
normal construction path runs. Timing is disabled by default and no
wall-clock threshold is added to ordinary CI.

This layer handles whole-frame reuse only. Incremental mutation and interaction
optimizations are introduced later in the stack.

Validation

• Steady-state reuse compared with explicitly forced rebuilding.
• Renderer phase counters confirm that eligible frames skip construction and
  planning work.
• Existing visual, picking, and selection results remain unchanged.