feat(P3): add GMSH Frontal-Delaunay tessellation engine

Introduces GMSH as an alternative to OCC BRepMesh for STEP→GLB tessellation.
GMSH produces conforming meshes that eliminate fan triangles at cylinder seam
edges — a structural limitation of OCC BRepMesh that cannot be fixed via
deflection parameters.

Changes:
- render-worker/Dockerfile: install gmsh>=4.15.0 + libglu1-mesa + libxft2
- export_step_to_gltf.py: --tessellation_engine occ|gmsh CLI arg +
  _tessellate_with_gmsh() using BRep→GMSH→Poly_Triangulation write-back
- admin.py: tessellation_engine setting (SETTINGS_DEFAULTS, SettingsOut,
  SettingsUpdate, validation)
- export_glb.py: pass tessellation_engine to export_step_to_gltf.py CLI in
  both geometry and production GLB tasks
- Admin.tsx: radio button UI for OCC vs GMSH selection

Tested: 121 faces meshed, 0 BRepMesh fallback, 649K triangles on sample part.
Clean seam edges for UV unwrap — GMSH respects B-rep periodic face boundaries.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

render-worker/Dockerfile

@@ -64,6 +64,12 @@ RUN pip3 install --no-cache-dir "cadquery>=2.4.0"
 # Install trimesh for STL→GLB geometry export (separate layer to avoid cache invalidation)
 RUN pip3 install --no-cache-dir "trimesh>=4.2.0"
 
+# libGLU + libXft required by gmsh's shared library loader
+RUN apt-get update && apt-get install -y --no-install-recommends libglu1-mesa libxft2 && rm -rf /var/lib/apt/lists/*
+
+# GMSH for Frontal-Delaunay tessellation (alternative to OCC BRepMesh)
+RUN pip3 install --no-cache-dir "gmsh>=4.15.0"
+
 # Copy render scripts
 COPY render-worker/scripts/ /render-scripts/