Add residual electron density (Fo−Fc) wireframe map to MoleculeWidget

[Copilot]

Compute and display a difference Fourier map from a CIF + SHELX HKL file, rendered as a green (positive) / red (negative) wireframe overlay in the existing 2D projection widget. Crystal symmetry is fully accounted for via gemmi's StructureFactorCalculatorX, which applies all space-group operators when computing Fc.

New: diff_map.py

• compute_diff_map(cif_path, hkl_path=None) — reads structure + ADPs from CIF, parses and merges SHELX HKL reflections (external file or _shelx_hkl_file embedded in CIF), computes Fc, estimates scale factor k = √(ΣFo²/ΣFc²), fills a ReciprocalComplexGrid with (Fo/k − |Fc|)·exp(iφ_c) coefficients, and inverse-FFTs to a real-space grid.
• get_mesh_segments(result, level_sigma=3.0) — marching cubes at ±N·σ → unique wireframe edges in Cartesian Å.

molecule2D.MoleculeWidget

• set_density_mesh(pos_segs, neg_segs) / clear_density_mesh() / show_density(bool) — manage the (N, 2, 3) segment arrays.
• Segments are rotated in-place alongside atoms during interactive rotation; drawn before atoms so the structure overlays the mesh.

loader.MoleculeLoader

• load_diff_map(hkl_path=None, level_sigma=3.0) — auto-detects <stem>.hkl / <stem>-finalcif.hkl, falls back to embedded CIF data.

viewer_widget.MoleculeViewerWidget

• Added Load Map button, Map σ spinbox (1–20, default 3.0), and Show Map checkbox.

Usage

loader = MoleculeLoader(widget)
loader.load_file("p31c.cif")
loader.load_diff_map(level_sigma=3.0)   # auto-detects p31c.hkl or embedded HKL

Dependencies

scikit-image added as an optional extra (pip install fastmolwidget[diffmap]). Raises ImportError with a clear message if absent.