fix(export): derive the ONNX output contract from num_out

[jayhesselberth]

Closes #357. The export sidecar always described the output as a single BCE logit, so a multiclass checkpoint shipped a contract telling consumers to apply a sigmoid to a [batch, num_out] softmax. The output block now comes from the checkpoint: binary exports keep the old block byte for byte; multiclass exports get [batch, num_out], a softmax convention, class order from label_map, and the calibration method when calibration.json sits beside the checkpoint. Export refuses to write a contract whose width disagrees with the graph, or whose label_map does not index exactly 0..num_out-1.

A multiclass checkpoint with no label_map still exports, with classes: null and a warning, since training can produce that shape and the column order is still correct.

https://claude.ai/code/session_01JDZiRc4dgzaA7nYFijxGbz