feat: step-based dock (usability plan, phase 3)

[lachlangrose]

Phase 3 of the usability plan. Stacked on #133: the base is feat/strat-column-ui, so retarget to main after #133 merges.

Changes

• The modelling dock has five steps: Data, Stratigraphy, Faults, Model, View. A step bar shows a status for each step (check, warning triangle, hollow circle). The footer shows the main message of the current step, with Back and Next.
• Each step has a check() function (gui/modelling/steps/checks.py, no QGIS import) that returns problems and to-do items.
• Save, Open, Reset and Settings are in the dock header.
• The map2loop dialogs open from the steps through map2loop_tools/launchers.py. They are not rewritten.
• Fault layers and the Fault Topology Calculator are in step 3.
• The toolbar has three actions (LoopStructural, 3D View, Help). The Plugins menu has a Tools submenu.
• separate_dock_widgets still works: step 5 has an Open 3D view button.

Notes

• The dock runs the checks every 750 ms while it is visible, because several widgets hold the only callback in the data manager.
• Not tested in a running QGIS: the connection stopped on plugin reload. Please run the dock before merge.

Tests

• 27 new unit tests for the checks (tests/unit/test_step_checks.py); all 97 unit tests pass.

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