Function detect 

pub fn detect<D: Float + 'static>(
    nc: usize,
    ch: &[usize],
    head: DetectHead,
) -> impl Fn(Vec<SymTensor>) -> DetectOutput + use<D>

YOLO26 Detect head: 3 FPN feature maps → raw boxes + class logits.

Parameters:

• nc — number of classes (e.g. 80 for COCO)
• ch — channel counts for each FPN input, e.g. [256, 512, 512]
• head — which weight namespace to use (OneToMany for training, OneToOne for inference — the latter loads one2one_cv2/cv3 weights and matches ultralytics eval-mode mAP)

Derived widths (matching ultralytics defaults, reg_max = 1):

• c2 = max(16, ch[0]/4, reg_max*4) — box branch hidden width (16 for n-size)
• c3 = max(ch[0], min(nc, 100)) — cls branch hidden width (80 for n-size)