feat(variation): render bounded confidence ellipsoid meshes

[d-sorgcodexagent]

Summary

• render actual default-off Gaussian position-content confidence-ellipsoid surfaces in PyQt6 and React variation 3D views
• enforce estimable/full-rank/exact-frame gating, deterministic bounded tessellation, immutable public mesh contracts, and linked filters/camera bounds
• replace raw-point camera materialization with streaming extrema that handles the supported 500×1,501 study scale
• add strict plot-definition schema v3 with exact v1/v2 migration while retaining parent feat(variation): expose confidence dispersion metrics #4415

Verification

• 1,460 Python and 868 React tests on approved mesh head
• max supported trace camera/render regression 85–254 ms without RangeError or raw point materialization
• 74 focused Python and 86 focused React tests on integrated head
• 5 real Chromium production-Worker workflows
• Python3.12/Mypy1.13, TypeScript/ESLint/Vite, Ruff/format, assertion policy, docs and size gates
• independent exact-head and integration GO reviews

Stack

Draft child of #4415. Cross-browser/AT certification, approved screenshots, plot-definition import UI, and remaining #4142 scope remain open.