rendering: reuse retained plan and resource work

[tritao]

Summary

• reuse render plans for opaque transform and non-opacity material updates
• preserve validated GPU resources across dynamic state-only updates
• keep transparent ordering, geometry, visibility, opacity, and structural invalidation conservative
• extend mutation benchmarks with backend phase timing

Performance

At 50k shared-recipe occurrences, transform/material update frames improve from roughly 41-46 ms to 16-18 ms on the tested Intel/Mesa hardware. Plan construction drops to zero and GPU-resource validation falls below 0.001 ms for eligible updates.

Validation

• 33 selected unit, GL core/compatibility, EGL, benchmark, viewer, parity, and stress tests
• FreeCADGui consumer build
• 106 focused 50k mutation curves with no unavailable cases

[tritao]

Superseded by #78. During the ownership restack, GitHub marked this PR merged because its rewritten head became an ancestor of its temporary base branch. No default-branch merge occurred; #78 contains the same rewritten layer at its correct position between #72 and #73.