feat(splat3d): render-depth certification P1 — DTO + scalar reference

Add `RenderDepthCertificate` (depth support interval z±k·σ_z with the seven
E_depth_total error terms kept separate for auditability) plus its scalar
reference `certify_depth_scalar`/`certify_batch_scalar`, `camera_depth_variance`
(Σ_cam[2][2] from scale+quat+view), and `screen_space_error` — the production
lift of cesium::sse::sse_for_tile (OGC 3D Tiles §7.4), which collapses to
geometricError·fy/distance for a pinhole camera.

Consumes the merged Camera/ProjectedBatch/Spd3 surface; no SIMD-dispatch
changes. 13 scalar reference tests (validation plan Tier 1).

https://claude.ai/code/session_017GFLBnDy23AWBqvkbHHC41

Author: claude