rendering: update retained frames incrementally

[tritao]

Builds on #71.

Purpose

Once a retained frame exists, a small scene edit should not require rebuilding
the whole frame or scanning every command path to discover what changed. The
renderer needs to know which commands depend on the changed node and which
derived data the edit can invalidate.

This PR introduces that mutation model. It indexes retained dependencies,
classifies scene notifications, and applies supported updates transactionally.
If an update cannot be proven safe, the manager falls back to a full rebuild.

What changed

• Index scene dependencies instead of rescanning every retained command path.
• Classify changes into explicit content, plan, resource, pick-topology, and
  rebuild invalidation domains.
• Patch transforms, legacy diffuse material state, visibility, and geometry
  through the internal SoRetainedUpdater.
• Apply mutations as a transaction so a failed patch restores the previous
  retained state before rebuilding.
• Keep SoRenderManager responsible only for orchestration: accept the
  incremental update or rebuild the frame.
• Report exact patch and rebuild counts in deterministic mutation workloads.

Ownership and fallback

SoRetainedUpdater is the sole classifier for supported scene mutations and
their invalidation effects. Unsupported nodes, ambiguous dependencies, or a
failed transaction take the existing full-rebuild path.

This layer defines invalidation semantics. Reusing plans and validated GPU
resources based on those guarantees is the responsibility of #78.

Validation

• Transform, material, visibility, and geometry mutation cases.
• Dependency-index and parent-occurrence handling.
• Transaction rollback and unsupported-mutation rebuild fallback.
• Deterministic assertions for command patches, plan/resource revisions,
  pick-topology invalidation, and full rebuild counts.