feat: model step and direct interpolation (usability plan, phase 4)

[lachlangrose]

Phase 4 of the usability plan. This PR is stacked on #134 (phase 3), so the base is feat/step-based-dock. Merge #134 first.

What changes

• One primary button replaces Initialize Model, Solve Model and Update Model Data. The text and action come from the model state (steps/build_plan.py). The problems of all visible steps show above the feature list.
• Build recalculates stale derived data. Out-of-date basal contacts, then thicknesses, are calculated on a background task with progress (main/derived_refresh.py). A failure stops the build and shows the error. With "Calculate from geology polygons" the model reads the extracted contacts directly. The project layer is for display only.
• Start choice in step 1: build from a geological map, or interpolate surfaces from constraints. The second hides steps 2 and 3. Saved with the state.
• Constraint types: value, interface, gradient/normal, tangent, form line, inequality, pairwise inequality. Each row has a weight and a Z source (layer, DEM or constant) (main/constraints.py). All interpolators take the same types, so none are hidden.
• Detach a feature that the stratigraphic column makes, to keep a fixed copy of its data. Rows that the user adds to a generated feature are now used in the build (before, they were ignored).
• Preview one feature as isolines on the DEM, as a temporary map layer (main/preview.py). Uses contourpy, which comes with matplotlib.
• Add Fault from a centre, strike, dip, pitch and size (main/parametric_fault.py). It replaces the old unused add_fault_dialog.ui with a dialog in code.

Not tested in QGIS

QGIS was not available where I wrote this. These are not run:

• the Qt code (primary button, dialogs, feature panel, preview layer, background tasks);
• the new QGIS tests: test_detach_feature.py, test_parametric_fault.py, and additions to test_manual_foliations.py and test_derived_data_state.py.

The unit tests pass (219) and ruff is clean on the new files. Please run the QGIS tests in CI and try the workflow by hand.

Things to check

• A model from constraint layers only, and a direct-constraint feature in a column model (the phase 4 acceptance test).
• A column reorder, then Build: the model uses contacts and thicknesses for the new order.
• The meaning of the three fault axes and the sign of the slip in LoopStructural. The tooltips state what the dialog means; the QGIS test checks only that the fault surface changes sign across the fault.
• Pairwise inequality: which group ends up above the other.
• contourpy is in the QGIS Python.

Limits

• Detach works only for column features, not faults.
• Parametric faults cannot be edited (delete and add again) and are not in the fault topology.
• The preview uses evenly spaced levels, not the unit boundary values. The "surface in the 3D view" option is not built.

🤖 Generated with Claude Code