Tier III: Perfect-model experiments (CESM2, MPI-ESM, GISS)

## Description
Test whether data-driven models can predict the future climate of existing ESMs when trained on their present-day output.

## Specification
- Train on ESM historical output (pre-2015), predict SSP2-4.5 future
- **CESM2** (ECS ≈ 5.2 K): required for ML models — hardest test
- **MPI-ESM** (ECS ≈ 3.63 K): extended
- **GISS ModelE2** (ECS ≈ 2.31 K): extended
- Evaluate using same Tier II metrics
- Models used for training must have been initialized before the test period (no data leakage)

## Required simulations
Perfect model × 3 ESMs (~85 years each)