EPIC: Model Zoo Expansion for spectraldiffx

This epic tracks the design, implementation, and documentation of a comprehensive model zoo for spectraldiffx — providing atmospheric and oceanic canonical benchmarks as JAX/NumPy-friendly code and demonstrative notebooks.

## Candidate Models & Sub-Issues
- Shallow Water Equations (SWE) on Sphere
- Flat-plane Barotropic QG
- QG on a Sphere
- Multi-layer QG (Baroclinic)
- Barotropic Vorticity Equation
- Kuramoto-Sivashinsky (1D)
- Passive Scalar Advection (e.g., Methane Plume)
- Stommel Gyre Model (Ocean)
- Beta-plane Turbulence
- Eady Model (Baroclinic Instability)
- CO₂/SST Tracer Advection
- Rayleigh-Bénard/Boussinesq Convection

Each subissue will detail the candidate formulation, required numerical operators, associated boundary/initial conditions, and dependencies (such as PRs needed for layers, spheres, or advanced coupling).

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Subissues each include: model formulation, equations, any forcing, starter code snippets, and dependencies/blockers.

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*Sub-issues will be opened and linked to this epic.*