[lachlangrose]

Summary

Phase 1 of the usability plan. This PR is stacked on #131 (base: fix/phase-0-quick-fixes).

• Shared layer roles (main/layer_roles.py): geology, geology_unit_field, fault_traces, structure, basal_contacts, dem, and a contacts source setting ("Calculate from geology polygons" or "Use a contacts layer"). They send a change event and are saved with the application state. Old state files load with default values.
• The Load Data widgets write the roles through the data manager. The Stratigraphic Column geology picker writes and follows the geology role.
• The Basal Contacts, Thickness, Sorter, Sampler and Paint Order dialogs show the roles as default layers. The user can still change a layer in a dialog for one run.
• Basal Contacts sets its new layer as the basal_contacts role and as the model contacts (only with the "Calculate from geology polygons" source).
• Derived-data record (main/derived_data.py): a hash of the inputs of the basal contacts, calculated thickness and strat_order field. A reorder marks them out of date. A colour change does not. A reorder and its undo leave them current.
• The Stratigraphic Column tab shows the out-of-date results with an Update button.
• Thickness source: each unit thickness is "typed" or "calculated". A calculated thickness does not replace a typed one.
• Docs: docs/usage/interface.md.

Notes

• The model build does not use the new records yet. That is phase 4.
• Update on basal contacts adds a new "Basal contacts" layer each run, like the dialog did before.

Test plan

• pytest tests in QGIS 3.42 (232 passed): unit tests for role storage, save/load, change event, input hash and thickness source
• Check in QGIS: select the geology layer one time and open each dialog; change the column order and see the Update message; move a unit back and see it disappear

🤖 Generated with Claude Code