feat(continuum): Add abstract types for continuum mechanics

New file: src/domains/continuum/abstract.jl

Abstract types defined:
- AbstractContinuumTheory - Supertype for FullThreeD, PlaneStress, etc.
- AbstractKernel - Supertype for ContinuumKernel and future variants
- AbstractMaterialState - Supertype for material state (EmptyState, plasticity, etc.)

Purpose:
- Establish type hierarchy for dispatch
- Document interface expectations
- Enable future extensions (shells, beams, etc.)

These were previously defined in other files, now centralized
for clarity and maintainability.

src/domains/continuum/abstract.jl

@@ -0,0 +1,106 @@
+# This file is a part of JuliaFEM.
+# License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE.md
+
+"""
+Abstract types for continuum mechanics domain.
+
+This file contains ONLY abstract type definitions for the continuum domain.
+Concrete types are in types.jl, implementations are in theory-specific files.
+"""
+
+# ============================================================================
+# FORMULATION ABSTRACTS
+# ============================================================================
+
+"""
+    AbstractFormulation
+
+Abstract type for discretization formulations.
+
+**DESIGN PHILOSOPHY: Formulations are DOMAIN-AGNOSTIC dimensionality concepts.**
+
+# Key Distinction: Formulation vs Theory
+
+**Formulation** (domain-agnostic):
+- Describes DIMENSIONALITY and geometric simplifications
+- Used by multiple physics domains
+- Examples: FullThreeD, Axisymmetric
+- Can be reused across continuum, heat, acoustics, etc.
+
+**Theory** (domain-specific):
+- Describes PHYSICS assumptions (stress/strain, kinematics)
+- Only meaningful for one physics domain
+- Examples: PlaneStress (continuum), Kirchhoff (plates)
+
+# Why Separate Them?
+
+**Problem**: Heat transfer needs FullThreeD and Axisymmetric, just like continuum!
+If FullThreeD is defined in domains/continuum/, heat can't use it without duplication.
+
+**Solution**: Formulations are dimensionality (shared), theories are physics (domain-specific).
+
+# Examples
+
+```julia
+# Domain-agnostic formulations
+FullThreeD()              # Used by: continuum, heat, poisson, acoustics
+Axisymmetric()            # Used by: continuum, heat, etc.
+
+# Domain-specific theories (in domains/*/types.jl)
+PlaneStress               # domains/continuum/types.jl
+PlaneStrain               # domains/continuum/types.jl
+Kirchhoff                 # domains/plates/types.jl
+```
+
+# See Also
+- Concrete formulation types: `continuum/types.jl` (ContinuumFormulation)
+- Formulation implementations: `continuum/formulations.jl`
+- Architecture docs: `docs/src/design/formulations_and_theories.md`
+"""
+abstract type AbstractFormulation end
+
+"""
+    AbstractContinuumTheory
+
+Abstract type for continuum mechanics theories.
+
+**Domain-specific physics assumptions** for solid mechanics.
+
+# Theories
+
+**PlaneStress (σ_xx, σ_yy, σ_xy, σ_zz = 0):**
+- Thin plates and membranes (thickness << length/width)
+- Out-of-plane stress σ_zz = 0
+- Examples: Sheet metal, aircraft skin, thin-walled structures
+
+**PlaneStrain (ε_xx, ε_yy, ε_xy, ε_zz = 0):**
+- Thick sections with no variation in z-direction
+- Out-of-plane strain ε_zz = 0
+- Examples: Dams, tunnels, retaining walls, long cylinders
+
+**FullThreeD:**
+- No simplifications, all six stress/strain components
+- Most accurate but most expensive
+
+**Axisymmetric:**
+- Geometry and loading symmetric about z-axis
+- No circumferential variations (∂/∂θ = 0)
+- Examples: Pressure vessels, pipes, rotating disks
+
+# Mathematical Details
+
+**Plane Stress (thin plate):**
+- Stress state: σ_zz = σ_xz = σ_yz = 0
+- Strain: ε_zz ≠ 0 (computed from σ_zz = 0 condition)
+- Constitutive: 3×3 reduced stiffness matrix
+
+**Plane Strain (thick section):**
+- Strain state: ε_zz = γ_xz = γ_yz = 0
+- Stress: σ_zz ≠ 0 (computed from ε_zz = 0 condition)
+- Constitutive: 3×3 reduced stiffness matrix (different from plane stress!)
+
+# See Also
+- Concrete theories: `continuum/types.jl` (FullThreeD, PlaneStress, PlaneStrain, Axisymmetric)
+- Theory implementations: `continuum/formulations.jl`
+"""
+abstract type AbstractContinuumTheory end