docs: Document integration test suite for diffusion model

Add comprehensive documentation for the first integration test suite in
OpenPFC, which validates the complete diffusion model simulation pipeline
against analytical solutions.

The test suite includes:
- 4 test cases with 331 assertions total
- 1D sinusoidal diffusion with convergence validation
- 3D Gaussian diffusion with spherical symmetry checks
- MPI consistency verification across parallel decompositions
- Test helper model demonstrating public field access philosophy
- All tests execute in under 2 seconds
- Validates spectral method accuracy (max error <1e-4, RMS <5e-5)

This establishes the pattern for future physics validation tests and
provides a safety net for refactoring core components.

Author: ahojukka5

CHANGELOG.md

@@ -9,6 +9,17 @@ SPDX-License-Identifier: AGPL-3.0-or-later
 
 ### Added
 
+- **Testing**: First integration test suite for diffusion model in
+  `tests/integration/test_diffusion_integration.cpp` validating complete
+  simulation pipeline (World → Decomposition → FFT → Model → Time Integration)
+  against analytical solutions. Includes 4 test cases with 331 assertions:
+  1D sinusoidal diffusion with convergence validation, 3D Gaussian diffusion
+  with spherical symmetry preservation, and MPI consistency verification.
+  Test helper model in `tests/fixtures/diffusion_model.hpp` demonstrates
+  "Laboratory, Not Fortress" philosophy with public `m_` prefixed fields.
+  All tests execute in <2 seconds validating spectral method accuracy
+  (max error <1e-4, RMS <5e-5). Establishes pattern for future physics
+  validation tests and enables safe refactoring of core components.
 - **World API**: Type-safe World construction using strong types from `strong_types.hpp`.
   Added new `create(GridSize, PhysicalOrigin, GridSpacing)` overload that prevents
   parameter confusion at compile time. Strong types (GridSize, PhysicalOrigin, GridSpacing)