Replace stale `Physics{...}` API docs with the actual 0.x role: a lightweight
trait root for `Elasticity`/`Thermal` tags and material dispatch, with legacy
`Physics` confined to `JuliaFEM.Legacy`.
- Shorten `AbstractPhysics` docstring and drop obsolete interface/examples.
- Define AbstractPhysics abstract type in dedicated file
- Consolidate documentation from previous duplicate definitions
- Document type as coupling of Mesh, Material, Field, and Formulation
- Add comprehensive examples for 3D solid, heat, and beam physics
- Include multiphysics pattern documentation
- Remove duplicate AbstractPhysics definitions across codebase
- 109 lines of documentation and abstract type definition
New file src/physics/abstract.jl defining physics system architecture:
- AbstractPhysics base type for all physics implementations
- get_unknown_field_name() returns primary field (displacement, temperature, etc.)
- get_formulation_type() returns :incremental, :total, or :rate
- get_unknown_field_dimension() returns DOFs per node
- assemble!() dispatch point for physics-specific assembly
- Comprehensive docstrings covering multi-physics coupling and GPU compatibility
- 138 lines documenting design philosophy and future extension
Jukka Aho