Split CACTS precision coverage so SP and DP run independently

[Copilot]

Motivation

Current CACTS configs define both EKAT_TEST_DOUBLE_PRECISION and
EKAT_TEST_SINGLE_PRECISION together, which forces Real=double in tests and
can mask single-precision regressions. This PR separates precision modes so
single-precision is exercised with only SP enabled, while release coverage
remains DP-only.

• Configuration split

  • Introduce precision-separated debug configurations (DP debug and SP debug).
  • Keep release configuration DP-only to match existing EAMxx practice.

• Workflow alignment

  • Update CI matrix/build-type selection to run the new CACTS build types.
  • Preserve existing CPU/CUDA job structure; only precision/build-type routing
    changes.

• Effective macro behavior

  • Ensure SP runs compile with only EKAT_TEST_SINGLE_PRECISION, avoiding
    accidental DP fallback from dual-definition builds.

# before (problematic for SP validation)
EKAT_TEST_DOUBLE_PRECISION: True
EKAT_TEST_SINGLE_PRECISION: True

# after (precision-isolated build types)
# debug-dp: EKAT_TEST_DOUBLE_PRECISION=True,  EKAT_TEST_SINGLE_PRECISION=False
# debug-sp: EKAT_TEST_DOUBLE_PRECISION=False, EKAT_TEST_SINGLE_PRECISION=True
# release:  EKAT_TEST_DOUBLE_PRECISION=True,  EKAT_TEST_SINGLE_PRECISION=False

E3SM Stakeholder Feedback

Issue feedback requests precision isolation in CACTS and recommends:
debug = both precisions (in separate builds), release = double precision only.
This PR implements that policy.

Testing

Coverage intent is now explicit in CI:

• debug DP run
• debug SP run
• release DP run

This ensures SP behavior is validated independently from DP.

[bartgol]

Copilot wants me to review the empty "Initial plan" commit?!? What a waste of a copilot credit...