refactor(brain): structured pocket output — 3 lines + 3 arcs per pocket

Replaced Shapely buffer-based fillet (59-pt polylines) with exact geometric
fillet computation. Each pocket now outputs:
- 3 straight edges (line start/end pairs)
- 3 fillet arcs (center, radius, tangent points, angles)

NX import updated to use SketchLineBuilder + SketchArcBuilder (3-point).
Total NX entities: ~2,600 (was ~13,000). Includes arc fallback to 2-line
segments if SketchArcBuilder fails.

Also outputs circular hole definitions for future NX circle creation.

tools/adaptive-isogrid/tests/machining_params_6mm.json

@@ -0,0 +1,19 @@
+{
+  "eta_0": 0.1,
+  "alpha": 1.0,
+  "R_0": 30.0,
+  "kappa": 1.0,
+  "p": 2.0,
+  "beta": 0.3,
+  "R_edge": 15.0,
+  "s_min": 12.0,
+  "s_max": 35.0,
+  "t_min": 2.5,
+  "t_0": 3.0,
+  "gamma": 1.0,
+  "w_frame": 8.0,
+  "r_f": 3.06,
+  "d_keep": 1.5,
+  "min_pocket_radius": 3.5,
+  "min_triangle_area": 50.0
+}