add run_visco_forward_sweep: viscoacoustic forward tool with attenuation switches, wired into selector + prompt

[aalzaki-gif]

Adds run_visco_forward_sweep — a sibling of run_forward_sweep for the
three-model ViscoAcoustic equation, so the agent can run an attenuating shot
from natural language.

• Three models: builds Q and omega as homogeneous grids, passes models=[vp, Q, omega].
• Exposes the two switches (phase_shift, amplitude_damping) as bool params —
  acoustic / phase-only / amplitude-only / both are all reachable.
• ViscoAcoustic imported directly (it's commented out of
  sweep.equations._equation_classes(), so list_equations doesn't show it).
• Wired into selector (CORE + forward + new visco group + fallback) and system prompt.
• Test asserts the switches change the gather (phase-only ≠ both).

Note: on MPS the amplitude term's FFT needs PYTORCH_ENABLE_MPS_FALLBACK=1 (CPU unaffected).