Add compas_3dec solver integration

[adellendice]

Summary

Integrate compas_3dec as a solver backend for compas_dem.

Changes

• Add Solver.ThreeDEC(...) configuration.
• Dispatch Problem.solve() to the ThreeDEC adapter.
• Convert a compas_dem.Problem with
  ThreeDECAnalysisBuilder.from_dem_problem(problem).build().
• Run the prepared analysis with ThreeDECSolver.
• Convert native 3DEC output through
  raw_results.to_compas_dem_results(analysis).
• Add solver configuration and dispatch tests.
• Add an opt-in real 3DEC gravity smoke test.
• Add a manual arch-gravity example with DEMViewer visualisation.

Verification

• Ruff formatting and linting pass.
• Automated adapter tests pass.
• Real 3DEC arch gravity analysis completes successfully.
• Converted DEM results display successfully in DEMViewer.

Notes

compas_3dec remains an optional dependency. The real solver smoke test is
skipped unless COMPAS_3DEC_EXECUTABLE is configured.

[tomvanmele]

if you add # doctest: +SKIP the awkward _ assignment is not necessary

[tomvanmele]

why?

[tomvanmele]

ha, i haven't switched this to mkdocs yet...