Fix a_w() to handle kappa==0 safely

[Copilot]

With the soluble_fraction approach, init_kappa() can set kpa to 0 (when soluble_fraction==0). The previous assert(kappa > 0) in a_w() would fire, and the denominator (rw3 - rd3*(1-kappa) - rd3_insol) collapses to (rw3 - rd3 - rd3_insol), producing 0/0 when there's no liquid water.

• Relax assertion from kappa > 0 to kappa >= 0
• Return 1.0 (pure water activity) when kappa == 0 and liquid water exists
• Existing early return of 0.0 already covers the rw3 <= rd3 + rd3_insol case

assert(kappa >= 0);
if ((rw3 - rd3 - rd3_insol) / si::cubic_meters <= std::nextafter(real_t(0), real_t(1)))
  return real_t(0.);
if (kappa == real_t(0))
  return real_t(1.);
return (rw3 - rd3 - rd3_insol) / (rw3 - rd3 * (real_t(1) - kappa) - rd3_insol);