Multi-layer figures (raster map, potential, current density, power
density) get a layer checkbox panel on interactive backends: unticking
a layer removes its row and the remaining rows grow to fill the window
(constrained layout reflows). At least one layer stays visible; saved
PNGs always contain every layer, with the panel hidden.
The four figure builders are restructured around per-layer painters
plus a finalize hook (legend/colorbar/annotations), driven by a shared
_layer_windows redraw loop; single-layer figures and non-interactive
runs are unchanged. The toggle path is exercised headlessly through
the real CheckButtons callback in the visual check.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
janik
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