Tier II: Seasonal cycle metrics (emergent constraints)

## Description
Evaluate seasonal cycle amplitude and phase as emergent constraints.

## Specification
- Mean annual temperature range over land (thermal damping/feedback strength)
- Seasonal amplitude of carbon uptake by land biosphere (CO2 fertilization constraint)
- Seasonal SST–low-cloud cover correlation (cloud-radiative feedback proxy for ECS)
- Computed over full historical period (not just post-2015 test window)

## Required variables
`tas`, `tos`, `clt`, `pr`

## Required simulations
historical (full period)

## References
Protocol §Tier II — Seasonal cycle