docs(physics): refocus AbstractPhysics as a tag marker

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.

src/physics/abstract.jl

@@ -4,106 +4,15 @@
 """
     AbstractPhysics
 
-Abstract type for all physics problems in JuliaFEM.
+Marker abstract type for physics category tags.
 
-Physics is the **coupling** of four components:
-- **Mesh**: Topology/geometry (where)
-- **Material**: Constitutive law (what material)
-- **Field**: Solution variables (what we solve for)
-- **Formulation**: Discretization strategy (how we discretize)
-
-# Interface Requirements
-
-All physics types must support:
-- `assemble!(physics)` - Assemble global system matrices/operators
-- `solve!(physics)` - Solve the physics problem
-- `add_dirichlet!(physics, ...)` - Apply essential boundary conditions
-- `add_neumann!(physics, ...)` - Apply natural boundary conditions
-
-# Concrete Types
-
-- `Physics{Formulation, Field, Mesh, Material}` - Standard FEM physics problem
-
-# Design Philosophy
-
-**Physics references Mesh (does not own it)**:
-- Multiple physics can share one mesh (multiphysics coupling)
-- No mesh duplication (memory efficient)
-- Mesh is the topology owner
-- Physics is the problem owner
-
-**Type parameters enable dispatch specialization**:
-```julia
-# Specialize assembly for formulation × field combinations
-assemble!(::Physics{ContinuumFormulation{FullThreeD}, Displacement{3}, M, Mat})
-assemble!(::Physics{BeamFormulation{Timoshenko}, DisplacementRotation{3}, M, Mat})
-
-# Generic fallback
-assemble!(::Physics{Fm, F, M, Mat}) where {Fm, F, M, Mat}
-```
-
-# Examples
-
-```julia
-# 3D solid mechanics
-physics = Physics(
-    name = "cantilever",
-    mesh = mesh,
-    element_set = :all,
-    field = Displacement{3}(),
-    formulation = ContinuumFormulation{FullThreeD}(),
-    material = LinearElastic(E=210e9, ν=0.3)
-)
-
-# Heat transfer
-physics_thermal = Physics(
-    name = "heat_conduction",
-    mesh = mesh,
-    element_set = :solid,
-    field = Temperature(),
-    formulation = ContinuumFormulation{FullThreeD}(),
-    material = ThermalMaterial(k=50.0, ρ=7850.0, c=450.0)
-)
-
-# 3D beam
-physics_beam = Physics(
-    name = "beam",
-    mesh = beam_mesh,
-    element_set = :all,
-    field = DisplacementRotation{3}(),
-    formulation = BeamFormulation{Timoshenko}(),
-    material = steel
-)
-```
-
-# Multiphysics Example
-
-```julia
-# Share one mesh between structural and thermal physics
-mesh = Mesh{Hex8}(nodes, connectivity)
-
-physics_structural = Physics(
-    name = "structure",
-    mesh = mesh,  # Reference, not copy!
-    field = Displacement{3}(),
-    formulation = ContinuumFormulation{FullThreeD}(),
-    material = steel
-)
-
-physics_thermal = Physics(
-    name = "thermal",
-    mesh = mesh,  # Same mesh reference!
-    field = Temperature(),
-    formulation = ContinuumFormulation{FullThreeD}(),
-    material = thermal_steel
-)
-```
-
-# See Also
-- [`assemble!`](@ref) - System assembly
-- [`solve!`](@ref) - Solve physics problem
-- [`add_dirichlet!`](@ref) - Essential BCs
-- [`add_neumann!`](@ref) - Natural BCs
-- Concrete type: `Physics` in `src/physics/types.jl`
+In the default 0.x build this is used purely for trait-based dispatch on lightweight
+type tags such as `Elasticity{Dim}` and `Thermal{Dim}` (see
+`src/physics/types.jl`), which materials use to declare what they support
+(`supported_physics`, `required_field_type`, `required_field_types`). It is
+not the parent type of any concrete `Physics{...}` struct in the active
+codebase. The older `Physics{Formulation, Field, Mesh, Material}` struct
+lives in `JuliaFEM.Legacy` and is loaded only when the user opts in via
+`JULIAFEM_ENABLE_LEGACY=1`.
 """
 abstract type AbstractPhysics end