Documented Debye-Huckel screening option cannot be built

Summary

SCREEN_METHOD=debye_huckel is documented and implemented in screen.H,
but GNU make rejects it. Adding the makefile selection alone also leaves
the screening test unable to compile because screen_name is missing.

Suggested priority: medium. This blocks a documented build configuration.

Affected code

• Make.Microphysics_extern:38-51: accepted screening methods.
• screening/screen.H:34-44: screening method name declarations.
• unit_test/test_screening_templated/main.cpp:154: uses screen_name.
• Docs/source/screening.rst: documents the standalone method.

Reviewed commit: e4b401b7637894a3db5ffa1e4b401509a332a1d6.
Related implementation: 66a8b4a8, PR #1688.

Reproduction

From the repository root:

cd unit_test/test_screening_templated
make -n SCREEN_METHOD=debye_huckel

The command exits with status 2:

../../Make.Microphysics_extern:50: *** Invalid value for SCREEN_METHOD.

The accompanying harness independently selects
-DSCREEN_METHOD=SCREEN_METHOD_debye_huckel at compilation. The screening
calculation works, but adding a reference to screen_name produces:

error: 'screen_name' was not declared in this scope

See run_harness.py, reproduce.cpp, and reproduction-output.txt in this
directory. This second check isolates the missing declaration without
editing the makefile.

Proposed fix

Add this branch before the rejecting else in Make.Microphysics_extern:

else ifeq ($(SCREEN_METHOD), debye_huckel)
  DEFINES += -DSCREEN_METHOD=SCREEN_METHOD_debye_huckel

Add the corresponding branch in the screen_name selection:

#elif SCREEN_METHOD == SCREEN_METHOD_debye_huckel
constexpr std::string_view screen_name = "debye_huckel";

Validation

Build and run unit_test/test_screening_templated with this method and
verify the output prefix is test_screening.debye_huckel. Add this option
to the screening build matrix so both omissions are covered.

Start with the SCREEN_METHOD selection in Make.Microphysics_extern:38-51 and the screen_name declarations in screening/screen.H:34-44. Run make -n SCREEN_METHOD=debye_huckel in unit_test/test_screening_templated, then use run_harness.py and reproduce.cpp to confirm the missing name declaration. Done means the test builds and runs with output prefix test_screening.debye_huckel and the method is covered by the screening build matrix.