rendering: reuse retained plan and resource work

[tritao]

Builds on #72.

Purpose

An incremental update can patch retained command content successfully while
leaving render-plan ordering and GPU geometry resources unchanged. Treating
every content revision as if it invalidated all derived work still forces plan
construction and backend resource validation on otherwise cheap transform or
material edits.

This PR consumes #72's explicit invalidation domains. It preserves a render
plan when planning semantics did not change and skips GPU resource validation
when resource identity and revisions remain valid.

What changed

• Reuse render plans for eligible opaque transform and non-opacity legacy
  material updates.
• Preserve validated GPU resources across dynamic state-only changes.
• Keep plan and resource revisions independent from the general content
  revision.
• Extend mutation benchmarks with the backend phases needed to prove which
  derived work was skipped.

Safety and ownership

This layer does not classify mutations. It relies on the invalidation result
produced by SoRetainedUpdater in #72. Transparent ordering, geometry,
visibility, opacity, pick topology, and structural changes remain
conservative and invalidate the derived work they can affect.

Performance

For 50,000 shared-recipe occurrences on the tested Intel/Mesa system,
transform and material update frames improved from roughly 41–46 ms to
16–18 ms. Eligible samples report zero plan-construction time and less than
0.001 ms of GPU-resource validation.

These measurements describe one controlled machine. Revision counters and
skipped-phase assertions are the deterministic contract.

Validation

• Full core-only and LegacyGL-enabled Coin test suites.
• Focused hardware smoke, interaction, and mutation benchmark modes.
• Parent GUI and Pivy build, link, and runtime import checks.

Stack note

This PR supersedes #77. GitHub marked #77 merged during an earlier ownership
restack because its rewritten head became an ancestor of a temporary base; it
was not merged into the repository's default branch.