fix(assemblers): use coords field instead of ξ in geometry cache

Update geometry cache to use coords field from QuadraturePoint
instead of deprecated ξ field.

- Change ip.ξ to ip.coords in create_geometry_cache function
- Update both gradient computation and detJ*w computation

src/assemblers/geometry_cache.jl

@@ -195,7 +195,7 @@ function create_geometry_cache(
     # Compute gradients and weights at all integration points
     ∇N_data_tuple = ntuple(NIP) do q
         ip = ips[q]
-        ξ = ip.ξ
+        ξ = ip.coords
         w = ip.weight
 
         # Compute Jacobian and physical gradients
@@ -220,7 +220,7 @@ function create_geometry_cache(
     # Compute detJ * weight
     detJ_w_tuple = ntuple(NIP) do q
         ip = ips[q]
-        ξ = ip.ξ
+        ξ = ip.coords
         w = ip.weight
 
         # Recompute Jacobian (could optimize by storing from above)