feat: integrate the FortFEM backend

[krystophny]

What changed

This PR moves the previously direct-to-main FortFEM integration into the
normal review workflow. It proposes the full seven-commit change set that:

• uses FortFEM for RT0 L2 integration and Maxwell solves;
• retains native FortFEM solve results and dispatches Maxwell solves to that backend;
• removes the FreeFem runtime and Triangle dependency from the native path;
• matches FreeFem nodal interpolation behavior; and
• adds adapter, dispatch, dependency-removal, and production-scale mesh tests.

Why

These changes were pushed directly to MEPHIT main without prior review.
Commit f3a3fc4 restores main to the exact pre-integration tree. This draft
PR reintroduces the same aggregate diff for explicit review before merge.

Impact

If approved and merged, MEPHIT gains the native FortFEM Maxwell/RT0 path and
removes the FreeFem/Triangle runtime requirements covered by this backend.
Until then, main retains the prior reviewed implementation.

Validation

• FortFEM is pinned in cmake/SetupFortfem.cmake
  at 605dc7f.
• A clean FetchContent configure resolved exactly that FortFEM revision.
• Native MEPHIT configure/build passed with the local tree; all 5 FortFEM
  adapter, production-mesh, dispatch, no-daemon, and no-Triangle tests passed.
• Legacy MEPHIT configure/build passed with the existing Triangle backend.
• The FortFEM upstream fpm test suite passed in full.