Update MESHFEM3D globe to give thin database information

[Rohit-Kakodkar]

Additive; do not touch save_arrays_solver.f90.

1. New writer src/meshfem3D/save_database_specfempp.F90, exposing
   save_database_specfempp(). Hook it in create_regions_mesh.F90 pass 2, at the
   same site as save_arrays_solver (~src/meshfem3D/create_regions_mesh.F90:397),
   gated by a new Par_file flag (e.g. SPECFEMPP_DATABASE = .true.). Register in
   rules.mk/Makefile.
2. Merge regions per rank. Accumulate per-rank element/node/boundary/adjacency
   data across the region loop and emit one file at the end, or write region blocks
   and merge in the reader. Prefer accumulate-then-emit for a clean single-mesh file.
3. Anchor extraction. Globe uses NGNOD = 27. Extract the 27 anchor nodes from
   the corner/edge/centre GLL positions of ibool (the inverse of
   hex_nodes/add_missing_nodes). Emit a deduplicated node list per rank
   (final geometry) and — when topo/ellip on — the reference-spherical anchors.
   To obtain reference anchors, capture the anchor coordinates before the
   topography/ellipticity calls in compute_element_properties (they are available
   as xelm/yelm/zelm right after Moho stretching + GLL interpolation).
4. Per-element context. Emit idoubling, rmin, rmax, elem_in_crust,
   region, medium_tag, property_tag. rmin/rmax are the rmins/rmaxs shell
   bounds already in scope; elem_in_crust is computed in
   compute_element_properties — thread it out to the writer.
5. Boundaries. Free surface (top of crust/mantle), CMB (bottom of crust/mantle
   / top of outer core), ICB (bottom of outer core / top of inner core), ocean-load
   faces. Source: ibelm_top/bottom per region + region adjacency.
6. Adjacency. Translate globe MPI interfaces (create_MPI_interfaces,
   my_neighbors_*, iboolleft/right_*) into the local CSR + MPI-neighbor format.
   Globe-native representation — no Cartesian re-encoding.
7. Dimensionalize coordinates (× R_PLANET) and write header constants.
8. Write MODEL_CONFIG. After read_compute_parameters the mesher already
   holds the resolved model selection (MODEL name, REFERENCE_1D_MODEL,
   THREE_D_MODEL, THREE_D_MODEL_IC, REFERENCE_CRUSTAL_MODEL) and the model
   flags in shared_parameters/meshfem_models_par. Serialize them into the
   header's MODEL_CONFIG block (§4.2) so SPECFEM++ can replay them into
   models_init without ever reading the globe Par_file.
9. Optional BAKED block. Reuse the values get_model already computes in
   pass 2 to append per-GLL material when material_mode == BAKED.