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>

.claude/settings.local.json

@@ -78,7 +78,10 @@
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       "mcp__Claude_in_Chrome__computer",
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+      "Bash(/c/Users/antoi/anaconda3/envs/atomizer/python.exe -m pip install:*)",
+      "Bash(/c/Users/antoi/anaconda3/envs/atomizer/python.exe tests/compare_triangle_vs_gmsh.py)",
+      "Bash(/c/Users/antoi/anaconda3/envs/atomizer/python.exe:*)"
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