correct normal for box minimum face in shape gradient

[groberts-flex]

@yaugenst-flex , this is the error I was mentioning this morning. I was going to include it in my boundary handling PR but just in case that takes a little bit to fully get in, I figured it was worth having this fix in sooner since it's needed for correct PEC handling.

Greptile Overview

Greptile Summary

This PR fixes a critical bug in the Box geometry's shape gradient computation where the normal vector for the minimum face was incorrectly pointing inward (+1) instead of outward (-1). The evaluate_gradient_at_points method requires outward-pointing normals for correct PEC (Perfect Electric Conductor) boundary handling in adjoint optimization.

Key changes:

• Modified tidy3d/components/geometry/base.py:2743 to conditionally set normal direction based on min_max_index
• When min_max_index == 0 (minimum face), normal now points in -1 direction (outward from lower bound)
• When min_max_index == 1 (maximum face), normal continues to point in +1 direction (outward from upper bound)
• Added appropriate changelog entry under "Fixed" section describing the impact

Confidence Score: 5/5

• This PR is safe to merge with minimal risk - it's a targeted bug fix with clear correctness improvement
• The fix is minimal, mathematically correct, and addresses a clear bug. The change correctly implements outward-pointing normals as required by the API documentation. The logic is straightforward: minimum faces need negative normals, maximum faces need positive normals. No other code paths are affected.
• No files require special attention

Important Files Changed

File Analysis

Filename	Score	Overview
CHANGELOG.md	5/5	Added clear, user-facing changelog entry describing the bug fix for Box shape gradient normal direction
tidy3d/components/geometry/base.py	5/5	Corrected normal vector direction for Box minimum face to point outward (-1 direction) as required by shape gradient API

Sequence Diagram

sequenceDiagram
    participant Client as Adjoint Client
    participant BoxFaces as Box._derivative_faces
    participant BoxFace as Box._derivative_face
    participant Eval as DerivativeInfo.evaluate_gradient_at_points
    
    Client->>BoxFaces: Compute shape gradients
    BoxFaces->>BoxFaces: Loop over min_max_index
    BoxFaces->>BoxFace: Call with min_max_index and axis_normal
    
    BoxFace->>BoxFace: Get coord from bounds_normal[min_max_index]
    BoxFace->>BoxFace: Build grid_points on face
    
    Note over BoxFace: Critical Fix: Set normal direction
    alt min_max_index equals 0
        BoxFace->>BoxFace: Set normals[:, axis_normal] = -1
    else min_max_index equals 1
        BoxFace->>BoxFace: Set normals[:, axis_normal] = +1
    end
    
    BoxFace->>Eval: Call with spatial_coords and normals
    Note over Eval: Requires outward-pointing normals for correct PEC handling
    Eval-->>BoxFace: Return gradient values
    
    BoxFace->>BoxFace: Compute vjp_value with integration_weight
    BoxFace-->>BoxFaces: Return vjp_value
    BoxFaces-->>Client: Return vjp_faces array

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Diff Coverage

Diff: origin/develop...HEAD, staged and unstaged changes

• tidy3d/components/geometry/base.py (100%)

Summary

• Total: 1 line
• Missing: 0 lines
• Coverage: 100%