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/src/brain/__main__.py

@@ -17,7 +17,8 @@ from src.atomizer_study import DEFAULT_PARAMS
 from src.shared.arc_utils import typed_segments_to_polyline
 from .density_field import evaluate_density_grid
 from .geometry_schema import normalize_geometry_schema
-from .triangulation import generate_triangulation
+from .triangulation_gmsh import generate_triangulation  # Gmsh Frontal-Delaunay (production)
+# from .triangulation import generate_triangulation  # Triangle library (fallback)
 from .pocket_profiles import generate_pockets
 from .profile_assembly import assemble_profile, profile_to_json
 from .validation import validate_profile