Add smoothstep node

[benstockil]

Add smoothstep node, as per OpenGL reference.

[gemini-code-assist]

Code Review

This pull request introduces a new smoothstep node in the math library to perform smooth Hermite interpolation. The review feedback points out a documentation error where edge_max is incorrectly described as mapping values above it to 0 instead of 1. Additionally, the feedback identifies a potential division-by-zero/NaN propagation issue when edge_min equals edge_max, and notes that the indentation of the new code should be updated to use tabs instead of spaces.

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[cubic-dev-ai]

2 issues found across 1 file

Confidence score: 3/5

• There is a concrete regression risk in node-graph/nodes/math/src/lib.rs: when edge_min == edge_max, the denominator becomes 0.0, yielding NaN that can propagate through clamp and downstream math.
• Given the severity (7/10) and high confidence, this is user-impacting behavior rather than a cosmetic issue, so merge risk is moderate until guarded.
• A secondary issue in node-graph/nodes/math/src/lib.rs is documentation mismatch for edge_max (docs say outputs map to 0, while smoothstep behavior maps above the upper edge to 1).
• Pay close attention to node-graph/nodes/math/src/lib.rs - add a zero-width-edge guard and align edge_max docs with actual smoothstep output.

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