feat(splat3d): render-depth certification P3 — HHTL depth cascade

Add HEEL→HIP→TWIG→LEAF block preselection (depth_cascade.rs) that fuses Cesium
SSE with the render-depth certificate: HEEL rejects behind near/far + off-screen
(scalar + F32x16 batch mask, parity-tested); HIP computes Cesium SSE + projected
radius; TWIG builds the depth certificate; LEAF fuses them — KeepCoarse only when
SSE ≤ max AND certificate passes AND occlusion confidence clears threshold, else
Refine / ProjectExact. This is Cesium's flat SSE rule extended with auditable
depth/ordering/occlusion uncertainty.

DTOs: BlockBounds, HhtlTier, HhtlAction, DepthCascadeBudget, BlockDepthDecision.
9 tests (per-tier decisions + SIMD/scalar HEEL parity). No dispatch changes.

https://claude.ai/code/session_017GFLBnDy23AWBqvkbHHC41

Author: claude