feat: separate the map and constraints workflows (phase 6.1)

[lachlangrose]

Summary

Phase 6.1 of the usability plan. With "Interpolate surfaces from constraints", the build now uses only the features that the user adds in step 4. The column, the contacts and the fault traces keep their data, but the model does not use it.

• The model manager has the workflow mode. update_model and refresh_feature_data skip the column and the fault traces. A column or topology change does not make the model stale. A mode change makes a built model stale.
• The data manager does not watch or check the map layers in this mode. The "processed" rows and sync_extra_constraints are empty. When the user changes back to "map", the map layers are read again.
• The "Select the geology layer" and "Select the structure layer" checks move from step 1 to the stratigraphy check.
• The build button says "Build model from constraints". Step 4 shows a note that the column and the map layers are not used.
• The plan has a new section, 6.5 (a model with only faults).

Tests

• Unit tests: tests/unit pass (233).
• New QGIS tests in tests/qgis/test_workflow_mode_model.py. I could not run them locally (no QGIS Python), so they need the QGIS test job.
• Checked by hand in QGIS: the build works in both modes.

This PR is stacked on #136.