Refactor: drop source_cell dependency on serialization_cereal (cell->lcao)

[Critsium-xy]

Background

Part of an incremental effort to untangle the source_* module dependency graph, so that lower layers (data structures) stop depending upward on higher layers (methods). See the module-layering discussion for the broader plan.

source_cell (a foundational L1 data-structure module) currently has several reverse/lateral edges into higher layers. This PR removes one of them: the direct source_cell -> source_lcao include edge.

What this changes

source/source_cell/bcast_cell.cpp included source_lcao/module_ri/serialization_cereal.h only to call ModuleBase::bcast_data_cereal on three ABFS file-name lists:

• exx_info.info_ri.files_abfs
• exx_info.info_opt_abfs.files_abfs
• exx_info.info_opt_abfs.files_jles

All three are plain std::vector<std::string>, so a full cereal (de)serialization round-trip is unnecessary here. This PR replaces those calls with a small local bcast_string_vector helper built on Parallel_Common::bcast_int / bcast_string — mirroring how ucell.orbital_fn is already broadcast a few lines above.

Behaviour

Unchanged: still a broadcast from rank 0 to all ranks. Only the transport mechanism (cereal binary archive over MPI_CHAR vs. per-string bcast_string) differs; the resulting data on every rank is identical.

Verification

-fsyntax-only passes in two configurations:

• default defines (no __EXX)
• with -D__EXX added

(The only warning emitted is a pre-existing one in source_base/math_erf_complex.h, unrelated to this change.)

Scope note

This removes the direct serialization_cereal.h include. bcast_cell.cpp still transitively reaches source_lcao/module_ri through exx_info.h (which itself pulls in conv_coulomb_pot_k.h); decoupling that is a separate, larger change and is intentionally out of scope here.