Fix out-of-grid fallback speed and degenerate GMM guard (#150)

Use the simulation's reference sound speed (not the default c0)
for geometric fallback on elements outside the grid. Add a guard
for near-constant parenchyma intensity that would cause the GMM
to degenerate.

Author: Brandon Kirkland

src/openlifu/bf/delay_methods/simulation_corrected.py

@@ -248,7 +248,7 @@ def fortran_linear_index(idx, shape):
             if el_i in out_of_grid:
                 # Element is outside the simulation grid; use geometric fallback
                 dist = np.linalg.norm(element_positions_m[el_i] - target.get_position(units="m"))
-                arrival_times[el_i] = dist / self.c0
+                arrival_times[el_i] = dist / sound_speed_ref
                 continue
 
             row = voxel_to_row[sensor_idx]

src/openlifu/seg/seg_methods/threshold_mri.py

@@ -909,6 +909,12 @@ def _classify_brain_gmm(
         # Extract parenchyma intensities for the initial fit.
         parenchyma_vals = classify_data[parenchyma_mask]
 
+        # Guard against near-constant intensity distributions where a
+        # 3-component GMM would be degenerate (all components collapse
+        # to the same mean, posteriors tie, argmax assigns class 0).
+        if float(np.ptp(parenchyma_vals)) < 1e-6:
+            raise ValueError("Parenchyma intensity range is near-zero; GMM cannot fit")
+
         # Initial GMM fit on raw parenchyma intensities.
         means, stds, weights = self._fit_gmm_1d(
             parenchyma_vals, n_components=n_components