fix: correct modified Huber loss gradient in ml/incr/binary-classification - #15317

Description

Fixes #13111. The else branch of _modifiedHuberLoss in ml/incr/binary-classification's Model was missing a factor of 2 in the "quadratically smoothed" region, and was being applied unconditionally for d = y*f(x) < 1 — including the d >= 1 region, where the modified Huber loss (and therefore its gradient) is exactly zero.

This goes slightly beyond what #13111 literally flagged (just the missing 2): I independently re-derived the three-region gradient against the cited references before changing anything, since applying only the factor-of-2 fix without also bounding the branch at d < 1.0 would still be wrong for d >= 1.

Per the modified Huber loss definition (Wikipedia, "Variant for classification"; scikit-learn's _sgd_fast.pyx.tp ModifiedHuber.dloss), the gradient of the loss w.r.t. the decision value p = y*f(x) is:

• p >= 1: 0
• -1 <= p < 1: -2*y*(1-p)
• p < -1: -4*y

Since the weight update here is w += eta * (-dloss/dp) * x, the middle region needs 2*eta*(y - p*y) (not eta*(y - p*y)), and the p >= 1 region needs no update at all. The p < -1 branch (4.0*eta*y) was already correct and is unchanged.

Testing

Added three new test cases to test/test.model.js covering each region (p < -1, -1 <= p < 1, p >= 1), confirming:

• the factor-of-2 fix in the quadratic region,
• the model weight vector is left unchanged when p >= 1,
• the existing p < -1 branch is unaffected.

$ node test/test.model.js
TAP version 13
# main export is a function
ok 1 ...
ok 2 main export is a function
# the modified Huber loss update includes a `2` factor when `-1 <= y*f(x) < 1` (see stdlib-js/stdlib#13111)
ok 3 includes the `2` factor in the update
# the modified Huber loss does not update the model weight vector when `y*f(x) >= 1`
ok 4 leaves the model weight vector unchanged
# the modified Huber loss update matches the existing `-4*eta*y` branch when `y*f(x) < -1`
ok 5 applies the unchanged `d < -1` branch

1..5
# tests 5
# pass  5

eslint on both touched files is clean (aside from a pre-existing, unrelated spelling warning on "pegasos").

Related Issues

Fixes #13111

Checklist

• Read, understood, and followed the contributing guidelines