Move the marginal-check numerics out of the GUI stage #17

What is wrong

The marginal-check stage of the GUI (croak/gui/stage_marginal_check.py) computes physics: _predict (trust-window taper, resampling onto a frequency grid, the predicted marginal and its centroid), _measured_moments, and the width-bracket judgement in _annotate_checks. AGENTS.md asks that GUI modules hold Qt glue, not numerics, and that plot data be prepared outside the GUI.

Why it matters

The same check cannot be run from a script or the CLI, and its numerics are tested only through the widget. A headless function would let croak.session report the check in a replayed session too.

What done looks like

A Qt-free function (in croak.marginal_checks, beside predicted_marginal) that takes the loaded data and the trust region and returns the measured and predicted curves, centroids and the width verdict; the stage only draws it and writes its summary. Tests move with it.

Context

Found while moving the stage's plots to pyqtgraph in #16, which left the computation where it was to stay in scope. Read, not run, beyond the existing tests.

Suggested labels: refactor, area: gui, area: preprocessing, priority: low.