fix(splat3d): address Codex review on render-depth certification (#206 follow-up)

[AdaWorldAPI]

Summary

Follow-up to the merged #206 fixing the two issues Codex flagged on commit de039e4c:

1. certify_batch_simd panic on short depth_var (P1). The SIMD lane load staged depth_var up to batch.len, so a caller passing a shorter buffer would index out of bounds and panic — contradicting the documented "does not panic on malformed input" and diverging from certify_batch_scalar's .get().unwrap_or(0.0). stage16 now reads via get().unwrap_or(0.0), so a short depth_var reads 0.0 for missing lanes (SIMD↔scalar parity preserved).
2. HEEL-rejected blocks returned a passing certificate (P2). cascade_block's reject branch called certify_depth_scalar(0,0,0,…), which yields passed = true under default params (and nonzero error terms under a nonzero budget) — a rejected block could be misread as certified. It now returns RenderDepthCertificate::ZERO (passed = false).

Promotes the shared zeroed/failed certificate to a public associated const RenderDepthCertificate::ZERO (used by the batch culled-slot path and HEEL rejection).

Tests

• certify_batch_simd_short_depth_var_does_not_panic — empty depth_var against a 20-splat batch; SIMD matches scalar, no panic.
• heel_rejects_behind_near_plane now asserts the rejected decision carries a failed certificate (!passed, zero covariance error).
• Full hpc::splat3d::depth* suite: 29 pass; cargo fmt --check clean.

https://claude.ai/code/session_017GFLBnDy23AWBqvkbHHC41

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Generated by Claude Code

Summary by CodeRabbit

• Bug Fixes

  • Fixed a regression where depth-rejected blocks could incorrectly appear as valid.
  • Improved robustness of depth variance handling to prevent crashes when data sizes are mismatched.

• Tests

  • Extended regression test coverage for depth certification edge cases.

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No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 1be8a18c-a701-4c6e-8bde-c4c722f13ba1

📥 Commits

Reviewing files that changed from the base of the PR and between 9ca4004 and b493999.

📒 Files selected for processing (2)

• src/hpc/splat3d/depth_cascade.rs
• src/hpc/splat3d/depth_cert.rs

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📝 Walkthrough

Walkthrough

This PR hardens depth-certificate handling in the splat-3D depth cascade and certification modules. A public RenderDepthCertificate::ZERO constant replaces internal constant patterns, used consistently across HEEL rejection, scalar batch certification, and SIMD batch certification for invalid/culled slots. SIMD stage16 now reads bounds-safely to prevent panics on short input buffers.

Changes

Depth Certificate Zero Constant and Rejection Path Hardening

Layer / File(s)	Summary
RenderDepthCertificate::ZERO constant definition src/hpc/splat3d/depth_cert.rs	Public associated constant ZERO represents a failed/zeroed certificate (passed = false) for culled or rejected slots, replacing prior file-local ZERO_CERT.
Scalar and SIMD certificate paths for invalid slots with robustness fix src/hpc/splat3d/depth_cert.rs, tests/*	Scalar and SIMD certificate generation emit RenderDepthCertificate::ZERO for invalid lanes. SIMD stage16 performs bounds-safe reads via get().unwrap_or(0.0) to prevent panics on short buffers. Test suite updated to assert against RenderDepthCertificate::ZERO and includes regression test verifying SIMD robustness.
HEEL rejection hardening and test src/hpc/splat3d/depth_cascade.rs	HEEL rejection path in cascade_block now assigns RenderDepthCertificate::ZERO instead of calling certify_depth_scalar with zero inputs. Test heel_rejects_behind_near_plane extended to verify rejected blocks have certificate.passed == false and certificate.covariance_depth_error == 0.0.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

Poem

🐰 A zero that's true, not a facade so neat,
Makes rejections and culls more complete!
No false-passing paths, just honest ZERO bright,
SIMD bounds-safe, and HEEL set right. ✨

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly references a specific fix addressing Codex review findings on render-depth certification, directly matching the primary changes across both modified files.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

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• Create stacked PR
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🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch claude/splat3d-cpu-simd-renderer-MAOO0

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