feat: Switch isogrid to Gmsh Frontal-Delaunay meshing (production default)

Replaces Triangle library with Gmsh as the default triangulation engine for
adaptive isogrid generation. Gmsh's Frontal-Delaunay algorithm provides:

- Better adaptive density response (concentric rings around holes)
- Superior triangle quality (min angles 30-35° vs 25-30°)
- Single-pass meshing with background size fields (vs iterative refinement)
- More equilateral triangles → uniform rib widths, better manufacturability
- Natural boundary conformance → cleaner frame edges

Comparison results (mixed hole weights plate):
- Min angle improvement: +5.1° (25.7° → 30.8°)
- Density field accuracy: Excellent vs Poor
- Visual quality: Concentric hole refinement vs random patterns

Changes:
- Updated src/brain/__main__.py to import triangulation_gmsh
- Added gmsh>=4.11 to requirements.txt (Triangle kept as fallback)
- Updated README and technical-spec.md
- Added comparison script and test results

Triangle library remains available as fallback option.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

tools/adaptive-isogrid/tests/lloyd_trial_params.json

@@ -0,0 +1,18 @@
+{
+  "lloyd_iterations": 5,
+  "s_min": 12.0,
+  "s_max": 35.0,
+  "t_min": 2.5,
+  "t_0": 3.5,
+  "gamma": 1.2,
+  "w_frame": 8.0,
+  "r_f": 1.5,
+  "d_keep": 1.5,
+  "eta_0": 0.1,
+  "alpha": 0.8,
+  "R_0": 40.0,
+  "kappa": 1.5,
+  "p": 2.0,
+  "beta": 0.3,
+  "R_edge": 15.0
+}