Replace Shewchuk Triangle with fortfem's MIT-licensed Triangle-compatible mesher

[krystophny]

Replaces Shewchuk's Triangle with fortfem's clean-room, MIT-licensed
triangle_compat mesher. Triangle's distribution restriction conflicts with
MEPHIT's MIT license.

Resolves #25

Changes

• Replaces the C Triangle wrapper with the Fortran
  mephit_triangulate adapter.
• Keeps the MEPHIT mesh contract: two constrained closed loops, one hole,
  fixed boundary segments, q20 quality refinement, and FreeFem .msh
  output.
• Removes the Triangle fetch and the old C implementation.
• Fetches only fortfem's dependency-free fortfem_triangle_compat target.

Dependency pin

Fortfem PR #56 is now
merged. This branch pins FortFEM main at
605dc7f.
FORTFEM_PATH can be set for local source testing.

Verification

• A clean configure fetched exactly 605dc7f056aa9b15cf8e0673eaa13b13ce12a273.
• The full MEPHIT mephit target built successfully with gfortran.
• On the 128-point inner and 16-point outer annulus fixture, the adapter
  produced 262 points, 380 triangles, 144 segments, and a 20.1546 degree
  minimum angle.
• The annulus area was 22763.8 in the fixture. No Triangle library is
  present in the resulting shared-library dependencies.

[krystophny]

Merged main; resolved conflicts in CMakeLists.txt (kept fortfem include dirs plus main's FFTW_INCLUDE_DIRS) and cmake/SetupCODE.cmake (kept main's hermetic-build overrides, dropped the Triangle override). #14 and #27 remain alternative TRIANGLE replacements to choose between.

[krystophny]

Closing for now; branch archived as z_archive/drop-triangle. It remains the reference implementation for the fortfem triangle_compat route in #25 and can be restored from the archived ref.