ECT fixes: DA cycling, spin-up caching, 1e-14 perturbation, GPU workflow

[cenamiller]

Summary

• DA cycling fix: Enable config_do_DAcycling on restart so theta perturbations propagate into the prognostic theta_m field, fixing bitwise-identical ensemble outputs
• Spin-up caching: Use a stable cache key for the 24h spin-up restart file, avoiding expensive re-runs on every commit
• Perturbation magnitude: Set to 1e-14 per the paper recommendation (Price-Broncucia et al. 2025); verified that theta is stored as float64 in restart files
• GPU subset workflow: New test-gpu.yml for NVHPC/MPICH/SMIOL cuda-vs-nogpu comparison on CIRRUS
• Misc: YAML parse fix for inline Python in composite action, Git LFS for restart push, Copilot review fixes

Validation

• 3-member test ensemble succeeded with 1e-14 perturbation (run #23611206904)
• Post-run theta means diverge across members, confirming perturbation propagation
• Ready for full 200-member ensemble generation once merged

[Copilot]

Pull request overview

Updates the ECT 120 km test-case configuration to use the paper-recommended perturbation magnitude, aligning the CI ECT setup with the documented methodology.

Changes:

• Set ECT_PERTURB_MAGNITUDE from 1e-5 to 1e-14 in the ECT 120 km config.env.

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