OpenPFC release v0.1.3 (2025-11-25)

This release focuses on extensibility, clarity, and zero-cost helpers:
custom coordinate systems via ADL, comprehensive API documentation with runnable
examples, FFT k-space helper functions, and an API cleanup moving
DiscreteField::interpolate() to a free function for consistency and ADL
extensibility.

Added: New custom coordinate system example, richer API documentation with
example programs, k-space helper functions for spectral methods, and a modern
interpolation API via free functions.

Fixed: none in this release.

Breaking changes: none — DiscreteField::interpolate() member remains available
but is deprecated in favor of the free function.

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Added

• Examples: Custom coordinate system example in examples/17_custom_coordinate_system.cpp
  demonstrating OpenPFC's extensibility via ADL (Argument-Dependent Lookup). Implements
  complete polar (2D: r, θ) and spherical (3D: r, θ, φ) coordinate systems with coordinate
  transformations (polar_to_coords(), polar_to_indices(), spherical_to_coords(),
  spherical_to_indices()). Includes comprehensive Doxygen documentation (615 lines),
  round-trip transformation verification, and 4-step recipe showing users how to add
  custom coordinate systems without modifying OpenPFC source code. Embodies "Laboratory,
  Not Fortress" philosophy - users can extend with cylindrical, spherical, or custom
  geometries through tag-based dispatch and free functions. Example compiles cleanly
  with zero warnings and demonstrates working coordinate conversions with correct output.
• Documentation: Comprehensive API documentation for top 10 most-used public
  APIs with detailed @example blocks and usage patterns. Enhanced documentation
  covers World (domain creation and coordinate transforms), Model (physics
  implementation), Simulator (time integration orchestration), FFT (spectral
  transforms), Time (time stepping), Decomposition (parallel decomposition),
  ResultsWriter (output formats), FieldModifier (IC/BC extensibility), and
  DiscreteField (coordinate-aware fields). Added 10 standalone example programs
  (4,570+ lines) demonstrating complete usage workflows from basic setup to
  production PFC simulations. Includes build system integration via
  docs/api/examples/CMakeLists.txt with BUILD_API_EXAMPLES option. Documentation
  warnings reduced from 9 to 1 (89% improvement). All examples validated and
  test suite confirms no regressions (73 test cases, 5,836 assertions passing).
• FFT: K-space helper functions in include/openpfc/fft/kspace.hpp providing
  zero-cost abstractions for wave vector calculations in spectral methods.
  Added 4 inline helper functions: k_frequency_scaling(world) for computing
  frequency scaling factors (2π/L), k_component(index, size, freq_scale) for
  wave vector components with Nyquist folding, k_laplacian_value(ki, kj, kk)
  for computing -k² Laplacian operator, and k_squared_value(ki, kj, kk) for
  magnitude squared. Eliminates 120+ lines of duplicated k-space calculation
  code across examples (04_diffusion_model.cpp, 12_cahn_hilliard.cpp, tungsten.cpp,
  etc.). All functions are inline, noexcept, and compile to identical machine
  code as manual implementation (zero runtime overhead). Comprehensive test
  coverage (177 assertions in 6 test cases). Example program added in
  examples/fft_kspace_helpers_example.cpp demonstrating before/after comparison.
• DiscreteField: Converted interpolate() from member function to free function
  pfc::interpolate(field, coords) aligning with OpenPFC's "structs + free functions"
  design philosophy. Added both mutable and const overloads for type safety. Member
  function deprecated with [[deprecated]] attribute for v1.x backward compatibility
  (will be removed in v2.0). Free function enables ADL-based extension allowing users
  to provide custom interpolation schemes without modifying OpenPFC. Updated all
  11 call sites across tests, examples, and documentation to use new API. Added
  comprehensive test coverage (95+ new test lines) including mutable/const overloads,
  ADL lookup verification, and nearest-neighbor rounding behavior tests. All 222
  assertions pass. Zero runtime overhead maintained (inline functions).