Fix cpp source flags in new API (#698)

* 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>

* Fix CppSimulation source flags to write signal length, not boolean

The C++ binary expects p_source_flag / u{x,y,z}_source_flag to contain
the number of time steps in the source signal (matching legacy
kWaveSimulation.source_p behavior), not just 0/1. Writing a boolean
caused the binary to reject valid HDF5 input files with time-varying
sources.

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

* Add comment clarifying misleading *_source_flag HDF5 field names

These fields carry the source signal length, not a boolean — the naming
is inherited from the C++ binary's HDF5 API.

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

* Use .shape[-1] instead of len([0]) for source flag values

Safer for both 1-D and 2-D source arrays and communicates intent
more clearly.

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

---------

Co-authored-by: Walter Simson <waltsims@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: waltsims

kwave/solvers/cpp_simulation.py

@@ -195,11 +195,14 @@ def _write_hdf5(self, filepath):
             "pml_x_size": pml_x_size,
             "pml_y_size": pml_y_size,
             "pml_z_size": pml_z_size,
-            "p_source_flag": int(has_p),
+            # NOTE: Despite the name "*_source_flag", the C++ binary expects
+            # the number of time points in the source signal (not a boolean).
+            # 0 = no source, >0 = number of time steps in the source signal.
+            "p_source_flag": np.asarray(source.p).shape[-1] if has_p else 0,
             "p0_source_flag": int(has_p0),
-            "ux_source_flag": int(has_ux),
-            "uy_source_flag": int(has_uy),
-            "uz_source_flag": int(has_uz),
+            "ux_source_flag": np.asarray(source.ux).shape[-1] if has_ux else 0,
+            "uy_source_flag": np.asarray(source.uy).shape[-1] if has_uy else 0,
+            "uz_source_flag": np.asarray(source.uz).shape[-1] if has_uz else 0,
             "sxx_source_flag": 0,
             "syy_source_flag": 0,
             "szz_source_flag": 0,