Formalize v0.6 extension contract

[miraflor]

Summary

Formalize the narrow downstream extension contract for the CGE-Core v0.6
public Hosoe workflow.

Contract

The protected lifecycle is:

CGE -> solve_benchmark() -> Equilibrium -> scenario() -> Scenario.set() -> Scenario.solve() -> Result

The contract also protects:

• public value() reads;
• repeated set() / solve() cycles on the same Scenario;
• immutability of previously returned Result snapshots;
• benchmark isolation;
• Result.compare() as the stable comparison path;
• public imports from cge_core and cge_core.models.

Public/private boundary

The contract deliberately does not include:

• private _engine or _snapshot state;
• raw Pyomo traversal;
• a universal Closure abstraction;
• plugin/backend registration;
• persistence or serialization guarantees;
• Scenario.unfix() as a required downstream capability;
• IFPRI or CAMCGE adaptation to the Hosoe facade;
• dynamic transition logic.

PyCGE remains available as the advanced/lower-level API, but its mutable
engine state is outside the downstream stability promise.

Changes

• add docs/developer/extension-contract.md;
• update the developer overview;
• add the extension contract to the docs TOC;
• add downstream contract regression tests.

Scope

This PR adds no new economic behavior and no new runtime abstraction. It
formalizes and tests capabilities already implemented in the v0.6 public API.

Validation

The Phase 5 contract tests and existing v0.6 documentation guards pass locally.

Merge policy

Do not merge until all CI checks are green.