Refactor: move read_orb from source_estate to source_cell (#7634)

[Critsium-xy]

Background

Fixes #7634.

source_cell is a data-structure layer that should not depend on the higher source_estate layer, yet read_atoms_helper.cpp called elecstate::read_orb_file(...), pulling source_cell back up onto source_estate (a circular dependency).

Root cause

read_orb_file is misplaced. It only:

• parses an orbital-file header and writes the results into Atom fields (nw, nwl, l_nchi, label_orb, Rcut) — all source_cell data;
• depends solely on source_cell/unitcell.h (same layer) and source_base/formatter.h (base layer).

Its only non-test caller is source_cell/read_atoms_helper.cpp; nothing in source_estate uses it. The elecstate:: namespace was a vestigial label — there is nothing electronic-structure about it.

Change

• Move read_orb.{h,cpp} from source_estate/ down into source_cell/.
• Rename the namespace elecstate → unitcell to match the module.
• Update the include paths and the two call sites in read_atoms_helper.cpp.
• Update every CMake target that compiled the file by its old path (source_cell/test, source_cell/test_pw, source_lcao/module_deepks/test, source_md/test, source_pw/module_pwdft/test) and the cell / estate object libraries.

After the change source_cell no longer #includes source_estate/...; read_orb only depends downward (source_base) and same-layer (unitcell.h), so no new cycle is introduced.

Verification

• Repo-wide grep confirms no remaining source_estate/read_orb or elecstate::read_orb_file references.
• g++ -std=c++14 -fsyntax-only -I source passes on both the moved read_orb.cpp and the caller read_atoms_helper.cpp.
• Behaviour is unchanged — this is a pure file move + namespace rename with no logic edits. MODULE_CELL_unitcell_test covers read_orb_file (two cases) and continues to build against the new location.