Prepare CGE-Core v0.6.0 release

[miraflor]

Summary

Finalize CGE-Core v0.6.0 release metadata, documentation, migration notes, and
release-consistency safeguards.

Changes

• bump pyproject.toml to 0.6.0;
• bump CITATION.cff to 0.6.0;
• update the source-tree __version__ fallback to 0.6.0;
• update the README software citation to 0.6.0;
• add the v0.6.0 changelog entry;
• document the migration from the lower-level PyCGE workflow to the public:
  • CGE
  • Equilibrium
  • Scenario
  • Result
    workflow;
• replace the stale notebook development-branch example with
  CGE_CORE_REF=v0.6.0;
• clarify PyCGE as the supported advanced/lower-level API;
• replace the inaccurate “Public domain API” facade wording with
  “Public scientific API”;
• add permanent release-consistency regression tests.

Release scope

v0.6.0 introduces the stable public Hosoe workflow:

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

The release also preserves:

• independent mutable scenarios from one solved benchmark;
• immutable previously returned Result snapshots;
• public value() reads;
• objective;
• Result.compare();
• public imports from cge_core and cge_core.models.

Compatibility

PyCGE remains supported as the advanced/lower-level API for:

• existing code;
• implementation inspection;
• direct engine use;
• advanced model development.

This release does not remove or deprecate PyCGE.

Deliberate non-changes

This PR does not:

• modify cge_core/engine.py;
• change economic equations;
• intentionally change benchmark or scenario numerical behavior;
• change IFPRI architecture;
• change CAMCGE architecture;
• introduce a universal closure abstraction;
• introduce plugin infrastructure;
• introduce persistence;
• add dynamic-model behavior.

Validation

The full public test suite passes locally.

git diff --check also passes.

The PR CI remains the final release gate for:

• Python 3.9–3.14 structural coverage;
• real IPOPT public solver coverage with zero skips;
• package build and wheel/sdist validation;
• Twine and CITATION.cff validation;
• Ruff correctness checks;
• documentation build.

Release policy

Do not merge until all CI checks are green.

After merge, tag the resulting main commit as v0.6.0 and create the
corresponding GitHub release.