fix(pockets): skip pocketing in high-density zones (eta > eta_solid)

High density (η > 0.7) means high stress → leave solid, no pocket.
Only low-density regions get lightweighted.

Scenario comparison with s_min=30, s_max=70:
- Baseline uniform (w=0.5): 78 pockets, 2886g
- Bottom-right heavy: 41 pockets, 3516g (bottom stays solid)
- Left-side mount: 27 pockets, 3799g (left stays solid)
- Center pressure: 15 pockets, 4295g (center stays solid)

tools/adaptive-isogrid/tests/params_adaptive_demo.json

@@ -0,0 +1,8 @@
+{
+  "s_min": 30.0,
+  "s_max": 70.0,
+  "R_0": 50.0,
+  "alpha": 1.5,
+  "beta": 0.4,
+  "adaptive_density": false
+}