refactor: rewrite triangulation using Triangle library (constrained Delaunay + quality refinement)

- Replace scipy.spatial.Delaunay with Shewchuk's Triangle (PSLG-based)
- Boundary conforming: PSLG constrains edges along inset contour + hole keepout rings
- Quality: min angle 25°, no slivers
- Per-triangle density-based area refinement (s_min=20, s_max=80)
- Clean boundary plotting (no more crooked v1 line resampling)
- Triangulation plot shows inset contour (red dashed) + keepout rings (orange dashed)
- Add sandbox2_brain_input.json geometry file

tools/adaptive-isogrid/src/atomizer_study.py

@@ -37,8 +37,8 @@ DEFAULT_PARAMS = {
     'p': 2.0,
     'beta': 0.3,
     'R_edge': 15.0,
-    's_min': 45.0,
-    's_max': 55.0,
+    's_min': 20.0,
+    's_max': 80.0,
     't_min': 2.5,
     't_0': 3.0,
     'gamma': 1.0,