Add CLAUDE.md for Claude Code onboarding (#696)

* Add CLAUDE.md for Claude Code onboarding

Provides build/test commands, architecture overview, and project-specific
naming conventions so future Claude Code sessions can be productive immediately.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* Update CLAUDE.md

Co-authored-by: greptile-apps[bot] <165735046+greptile-apps[bot]@users.noreply.github.com>

* Update CLAUDE.md

Co-authored-by: greptile-apps[bot] <165735046+greptile-apps[bot]@users.noreply.github.com>

---------

Co-authored-by: Walter Simson <waltsims@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Co-authored-by: greptile-apps[bot] <165735046+greptile-apps[bot]@users.noreply.github.com>

Author: 3 people

CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## What is k-wave-python?
+
+Python implementation of the [k-Wave](http://www.k-wave.org/) acoustics toolbox for time-domain acoustic and ultrasound simulations. Two backends: a pure Python/NumPy/CuPy solver and C++ binaries (OMP/CUDA).
+
+## Commands
+
+```bash
+# Setup
+uv sync --extra dev --extra test
+uv run pre-commit install
+
+# Run tests
+uv run pytest                              # all tests
+uv run pytest tests/test_kgrid.py          # single file
+uv run pytest tests/test_kgrid.py::test_name  # single test
+uv run pytest -m integration               # MATLAB-reference tests only
+
+# Lint/format
+uv run ruff check . --fix
+uv run ruff format .
+uv run pre-commit run --all-files
+
+# Run an example
+uv run examples/ivp_homogeneous_medium.py
+
+# Build
+uv build
+```
+
+Always use `uv run` (not `uv run python`) for pytest, examples, and scripts.
+
+## Code Style
+
+- Line length: 140 (configured in `pyproject.toml` under `[tool.ruff]`)
+- Pre-commit hooks: ruff (lint + format), codespell, nb-clean
+- Ruff ignores F821 (nested function refs) and F722 (jaxtyping annotations)
+- Type annotations use `beartype` + `jaxtyping`
+
+## Architecture
+
+### Entry point
+
+`kwave/kspaceFirstOrder.py` — `kspaceFirstOrder()` is the unified API. It accepts `kgrid`, `medium`, `source`, `sensor` and dispatches to the appropriate backend.
+
+```
+kspaceFirstOrder(kgrid, medium, source, sensor, backend="python"|"cpp", device="cpu"|"gpu")
+```
+
+### Two backends
+
+- **Python backend** (`kwave/solvers/kspace_solver.py`): `Simulation` class implementing k-space pseudospectral method in NumPy/CuPy. Supports 1D/2D/3D. Uses CuPy for GPU when `device="gpu"`.
+- **C++ backend** (`kwave/solvers/cpp_simulation.py` + `kwave/executor.py`): Serializes to HDF5, invokes compiled C++ binary, reads results back. `save_only=True` writes HDF5 without running (for cluster jobs).
+
+### Core data classes
+
+- `kWaveGrid` (`kwave/kgrid.py`) — domain discretization, spacing, time array
+- `kWaveMedium` (`kwave/kmedium.py`) — sound speed, density, absorption, nonlinearity
+- `kSource` (`kwave/ksource.py`) — pressure/velocity sources with masks and signals
+- `kSensor` (`kwave/ksensor.py`) — sensor mask and recording configuration
+
+### Legacy path
+
+`kspaceFirstOrder2D()` / `kspaceFirstOrder3D()` in their respective files route through `kWaveSimulation` (`kwave/kWaveSimulation.py`) — a large legacy dataclass used by the C++ backend path. New code should use `kspaceFirstOrder()` directly.
+
+### PML handling
+
+When `pml_inside=False` (default), `kspaceFirstOrder()` expands the grid by `2*pml_size` before simulation and strips PML from full-grid output fields afterward. `pml_size="auto"` selects optimal sizes via `get_optimal_pml_size()`.
+
+### Key utilities
+
+- `kwave/utils/pml.py` — PML sizing (`get_pml()`, `get_optimal_pml_size()`)
+- `kwave/utils/mapgen.py` — geometry generators (`make_disc`, `make_ball`, `make_cart_circle`, etc.)
+- `kwave/utils/signals.py` — signal generation (tone bursts, filtering)
+- `kwave/utils/filters.py` — spatial smoothing, Gaussian filters
+- `kwave/utils/io.py` — HDF5 read/write
+- `kwave/utils/conversion.py` — unit conversion, `cart2grid`
+
+## Testing
+
+- Tests in `tests/`, configured via `[tool.pytest.ini_options]` in `pyproject.toml`
+- Integration tests (`@pytest.mark.integration`) compare against MATLAB reference data
+- Test fixtures in `tests/integration/conftest.py`: `load_matlab_ref` (fixture), `assert_fields_close` (helper function for field comparison)
+- MATLAB reference data for `@pytest.mark.integration` tests lives in `tests/matlab_test_data_collectors/python_testers/collectedValues/` (hardcoded in `conftest.py`)
+- `test_example_parity.py` (MATLAB-parity tests) reads `KWAVE_MATLAB_REF_DIR` (defaults to `~/git/k-wave-cupy/tests` if unset)
+
+## Naming Conventions
+
+- `backend="python"` or `"cpp"` (not "native")
+- `device="cpu"` or `"gpu"` (not `use_gpu` bool)
+- `quiet=True` to suppress output; `debug=True` for detailed output
+- `pml_size="auto"` for automatic PML sizing (not a separate boolean)