Stable serialization: VolSurface.save/load with a versioned JSON schema #23

Context

Calibrating is expensive; evaluating is cheap. Production systems calibrate once and distribute the surface. Pickle is not a schema.

Proposal

surface.save(path) / VolSurface.load(path) with a stable, versioned JSON schema: schema_version, svi-py version, model name, per-slice maturities/parameters/forwards, rate, interp_method, and the fit report/provenance when present. Round-trips exactly; load validates schema_version and model availability.

Acceptance criteria

• save -> load round-trip reproduces evaluation bitwise
• Schema documented; unknown schema_version rejected with a clear error
• Works for all seven parametrizations