Refactor Slice 3B: Move build logic into `CIME/core/build/`

Part of #4950. Slice 3B of 5. Estimated 5 weeks. Depends on #4998.

## Goal

Move build logic from `build.py` (1350 lines) into focused modules under
`CIME/core/build/`. Keep `CIME/build_scripts/` as the stable external wrapper.

## Tasks

### Extract into `CIME/core/build/`
- [ ] Build planning logic.
- [ ] Build execution / orchestration.
- [ ] `build.py` becomes a thin re-export layer.

### Move from `utils.py`
- [ ] `analyze_build_log`, `run_bld_cmd_ensure_logging`,
`copy_local_macros_to_dir` → `core/build/`.
- [ ] `safe_copy`, `safe_recursive_copy`, `symlink_force`, `copy_globs`,
`copy_over_file`, `copyifnewer` → `CIME/core/fileops.py` (build is heavy
consumer, pulled here).

### Consolidation — free functions taking `case`
- [ ] `case_build()`, `clean()`, `_clean_impl()`, `post_build()` (`build.py`)
currently access `case._gitinterface` (private!), call `case.get_value`,
`case.set_value`, `case.flush()`. Make them `Case` methods that delegate
to `core/build/`.
- [ ] `get_standard_cmake_args()`, `get_standard_makefile_args()`,
`uses_kokkos()` (`build.py`) extract ~15 values from Case.
Consolidate as Case methods or a `BuildConfig` data class.
- [ ] `generate_makefile_macro()` (`build.py`) calls `case.get_value()` —
same pattern.

### Deduplicate copy functions
- [ ] Four overlapping copy functions in `utils.py` (`safe_copy`,
`copy_over_file`, `copyifnewer`, `copy_globs`) → coherent
`CIME/core/fileops.py` API.

### Sharedlibs (resolves #4837)
- [ ] Sharedlibs list currently hard-coded; expose as configurable.

### Imports & re-exports
- [ ] Update internal CIME imports to `core/build/`.
- [ ] Keep re-exports in `build.py` and `utils.py`.
- [ ] `CIME/build_scripts/` unchanged — stable external interface.

## Definition of done

- [ ] All build tests pass.
- [ ] `CIME/build_scripts/` compatibility maintained.
- [ ] External models (E3SM, CESM, NorESM) build unchanged.
- [ ] No private-attribute access (`case._gitinterface` etc.) from outside Case.
- [ ] One coherent fileops API; no four-copy-function-zoo.
- [ ] Resolves #4837.

## Related

- Resolves #4837 (sharedlibs hard-coding)
- Depends on #4998 (config loading must be stable first)
- Blocks #5000 (Case build delegation comes from this slice)

Start by reading build.py, utils.py, CIME/core/fileops.py, CIME/build_scripts/, and the dependent issue #4998; run the existing build tests before changing structure. Trace the listed Case access patterns and current copy functions, then verify that build tests pass, build_scripts and external model builds remain compatible, private Case attributes are isolated, and sharedlibs is configurable.

Written by the indexing model from the issue text.