Drop a duplicated example line and remove the commented-out Fluid placeholder
that suggested unfinished velocity dispatch.
- Clarify `Thermal{Dim}` wording; delete stale `Fluid{Dim}` sketch.
Replace monolithic Physics struct and boundary condition types with
simpler physics category types used for trait-based dispatch.
- Remove Physics struct (Formulation, Field, Mesh, Material coupling)
- Remove DirichletBC and NeumannBC boundary condition storage types
- Remove Constraint type
- Add Elasticity{Dim} and Thermal{Dim} physics category types
- Add required_field_type trait function for physics-to-field mapping
- Simplify to type tags for material trait dispatch
- Support compile-time field type inference from physics
- Align with new architecture: physics types are dispatch tags, not problem containers