Remap optimizations

[jacob-moore22]

Description

Extracts the final optimization stage (REMAP_OPT=6) out of the experimental
remap_dg_lumped_test_cuda.cpp into a clean, standalone implementation at
examples/reference_element/src/remap_dg_lumped_optimized.cpp, and factors the
3x3 Jacobian math that was duplicated across its kernels into reusable inline
Kokkos helpers.

remap_dg_lumped_test_cuda.cpp carried six cumulative optimization levels behind
#if REMAP_OPT >= N, plus scaffolding that only existed for experimentation. None
of it was wired into the build. This PR keeps only the level-6 code path and drops
the scaffolding, taking the file from 2263 to 1503 lines while reproducing the
reference results exactly.

Optimizations retained (all previously validated at level 6):

• flat 1D index spaces instead of the FOR_FIRST/FOR_SECOND team policy
• Jacobian, determinate and inverse computed once per quadrature point in a
  dedicated pass rather than once per DOF
• field/velocity reconstruction hoisted out of the DOF loop
• reference-element tables replicated in the layouts the kernels read, so a warp
  walks contiguous doubles
• register-resident surface Jacobian and a fused single-pass L1/L2 error kernel
• no host fences between stream-ordered kernels

New shared helpers in src/geometry/geometry.h, taking 9 doubles rather than
views, as the natural counterpart to the existing scalar det_3x3:

• invert_3x3(det, j00..j22, i00..i22) — the scalar Cramer inverse that
  cramers_rule.hpp was missing (it has view-based forms only). This removes a
  ~30-line block that was copy-pasted verbatim between build_element_geometry
  and build_surface_flux.
• nanson_area_normal(...) — reference normal to physical area normal

How Has This Been Tested?

The clean version is compared against remap_dg_lumped_test_cuda.cpp built with
-DREMAP_OPT=6, which is the exact code it was extracted from, so the two are
expected to agree bit-for-bit rather than merely to within a tolerance.

This was done on serial, OpenMP, and Cuda builds, and all passed.