Phase 8: Lowercase remaining top-level directories (mapl3g/, Python/, Tests/)

[tclune]

Summary

Phase 8 (clean-up) of the MAPL v3 directory restructuring (#4905).

Work Items

1. Rename mapl3g/ → mapl/

• git mv mapl3g/ mapl/
• Update top-level CMakeLists.txt: add_subdirectory(mapl3g) → add_subdirectory(mapl)
• No CMake target name changes needed (esma_set_this(OVERRIDE MAPL) already in place)

2. Rename Python/ → python/

• git mv Python/ python/
• Update top-level CMakeLists.txt: add_subdirectory(Python) → add_subdirectory(python)
• Update python/CMakeLists.txt: install path lib/Python → lib/python

3. Rename Tests/ → tests/

• git mv Tests/ tests/
• Update top-level CMakeLists.txt: add_subdirectory(Tests) → add_subdirectory(tests)
• Update tests/CMakeLists.txt: install path ${esma_include}/Tests → ${esma_include}/tests

Deferred

• MAPL_ESMF.F90 rename (ESMF_Subset.F90 / mapl3g_ESMF_Subset module) — separate issue
• docs/ already lowercase — no-op
• base3g/ deletion — handled separately
• MAPL2 legacy umbrella — handled separately

References

• Parent: Implement MAPL v3 directory & library restructuring #4905
• Previous phase: Phase 7: Rename Apps/ → apps/ and apply programs/ convention (closes #4905 phase 7) #4940

[tclune]

The MAPL.shared CMake target has been replaced by a backward-compatibility INTERFACE alias pointing to MAPL.mp_utils (which transitively pulls in MAPL.utils). The alias must be removed in a follow-on PR once all consumer repos (GEOSgcm, etc.) have been updated to link against MAPL.utils or MAPL.mp_utils directly.