…ic columns

Guards against the swap fixed in 814be12: model_manager.py's per-unit
training value and LoopStructural's get_isovalues() must agree on
which direction values increase, or extracted isosurfaces get labelled
with the wrong unit while keeping correct geometry.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

# Conflicts:
#	tests/qgis/test_stratigraphic_value_consistency.py

Look up each unit's colour from the stratigraphic column and attach it
to the extracted basal contacts, both from the widget and the
processing algorithm, so contact lines are styled to match the units
they belong to and are easier to tell apart on the map canvas.

Changing nelements, regularisation, contact/orientation point weights,
or fault parameters in a feature details panel used to schedule a
debounced rebuild that silently re-solved the feature 500ms later.
Solving can be expensive, so instead just flag the feature as not up
to date (its tick turns red) and leave the actual solve to the
user-initiated "Solve Model" action.

- to_dict() crashed with RuntimeError on save if a basal contacts,
  fault traces, or structural orientations layer had been deleted
  from the project while still referenced; now caught like the
  existing dem_layer guard, and dem_layer_name is initialised so a
  caught DEM error can't leave it undefined either.
- AllSampler returned a columnless DataFrame when a fault trace
  layer had no sampled points, causing a KeyError on 'feature_id'
  downstream in update_fault_points.

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>