Improve near-unity validation message; skip unused nabla1 computation

- Error message now distinguishes Python vs C++ backend workarounds
- nabla1/tau only computed when absorption is actually enabled

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

Author: waltsims

kwave/solvers/kspace_solver.py

@@ -427,9 +427,12 @@ def _setup_physics_operators(self):
                 self._absorption = lambda div_u: 0 if no_absorption else -2 * alpha_np * self.c0 * self.rho0 * div_u
                 self._dispersion = lambda rho: 0
             else:  # Power-law with fractional Laplacian
-                tau = -2 * alpha_np * self.c0 ** (alpha_power - 1)
-                nabla1 = self._fractional_laplacian(alpha_power - 2)
-                self._absorption = (lambda div_u: 0) if no_absorption else (lambda div_u: tau * self._diff(self.rho0 * div_u, nabla1))
+                if no_absorption:
+                    self._absorption = lambda div_u: 0
+                else:
+                    tau = -2 * alpha_np * self.c0 ** (alpha_power - 1)
+                    nabla1 = self._fractional_laplacian(alpha_power - 2)
+                    self._absorption = lambda div_u: tau * self._diff(self.rho0 * div_u, nabla1)
 
                 if no_dispersion:
                     self._dispersion = lambda rho: 0

kwave/solvers/validation.py

@@ -52,8 +52,9 @@ def validate_medium(medium, kgrid):
                 raise ValueError(
                     f"medium.alpha_power={power} is too close to 1.0. The dispersion term "
                     f"contains tan(pi*alpha_power/2), which diverges at alpha_power=1. "
-                    f"Set medium.alpha_mode='no_dispersion' to disable the dispersion term, "
-                    f"or use an alpha_power value further from 1.0."
+                    f"For backend='python': set medium.alpha_mode='no_dispersion' to disable "
+                    f"the dispersion term. For backend='cpp' or to use both absorption and "
+                    f"dispersion, choose an alpha_power value further from 1.0."
                 )