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>

frontend/src/pages/Admin.tsx

@@ -112,6 +112,7 @@ export default function AdminPage() {
     gltf_preview_angular_deflection: number
     gltf_production_linear_deflection: number
     gltf_production_angular_deflection: number
+    tessellation_engine: string
   }
 
   const { data: settings } = useQuery({
@@ -1459,6 +1460,34 @@ export default function AdminPage() {
             )
           })()}
 
+          {/* Tessellation engine selector */}
+          <div className="space-y-2">
+            <p className="text-xs font-semibold text-content-secondary uppercase tracking-wide">Tessellation Engine</p>
+            <div className="flex items-start gap-4">
+              <div className="flex flex-col gap-2">
+                {[
+                  { value: 'occ', label: 'OCC BRepMesh', description: 'Default engine. Fast, but produces fan triangles at cylinder seam edges.' },
+                  { value: 'gmsh', label: 'GMSH Frontal-Delaunay', description: 'Conforming mesh — no fan triangles on cylinders. +10–30% export time. Recommended for cylindrical parts.' },
+                ].map(opt => (
+                  <label key={opt.value} className="flex items-start gap-3 cursor-pointer p-3 rounded-lg border border-border-default hover:border-blue-400 transition-colors">
+                    <input
+                      type="radio"
+                      name="tessellation_engine"
+                      value={opt.value}
+                      checked={(tess.tessellation_engine ?? 'occ') === opt.value}
+                      onChange={() => setTessellationDraft(d => ({ ...d, tessellation_engine: opt.value }))}
+                      className="mt-0.5 shrink-0"
+                    />
+                    <div>
+                      <div className="text-sm font-medium">{opt.label}</div>
+                      <div className="text-xs text-content-muted mt-0.5">{opt.description}</div>
+                    </div>
+                  </label>
+                ))}
+              </div>
+            </div>
+          </div>
+
           {/* Manual inputs */}
           <div className="grid grid-cols-2 gap-6">
             <div className="space-y-4">