Clean up v0.6 public-facing internals

[miraflor]

Summary

Complete the conservative internal cleanup for CGE-Core v0.6 after the public
API, outward-facing surfaces, and downstream extension contract have been
stabilized.

Changes

• migrate remaining first-time-user documentation to the public v0.6 workflow;
• update the OG-Core crosswalk to lead with:
  • CGE
  • solve_benchmark()
  • Equilibrium
  • Scenario
  • Scenario.set()
  • Scenario.solve()
  • Result
  • Result.compare();
• update the public samtools example to use CGE and StdCGE;
• retain the existing lower-level PyCGE SAM validator internally;
• replace “legacy” terminology in the facade with “lower-level” to reflect
  PyCGE's continued supported status;
• replace implementation-class wording such as StdModelDef in Control Room
  user-facing text with scientific model terminology;
• clarify project provenance and credit James Matthew Miraflor as
  Project Lead and Maintainer;
• distinguish project leadership and maintenance from authorship of inherited
  PyCGE code and underlying model specifications;
• add Phase 6 regression guards.

Deliberate non-changes

This PR does not:

• modify cge_core/engine.py;
• change economic equations;
• remove or deprecate PyCGE;
• rename the internal StdModelDef or SplModelDef classes;
• change IFPRI;
• change CAMCGE;
• add a universal closure abstraction;
• add persistence or plugin infrastructure;
• change dynamic-model behavior;
• bump the release version.

PyCGE remains the supported advanced/lower-level API in v0.6.

Validation

The Phase 6 cleanup guards and the existing v0.6 documentation, extension
contract, and Control Room regression tests pass locally.

git diff --check also passes.

Scope

This is a terminology, documentation, provenance, and outward-facing cleanup.
It introduces no new economic or solver behavior.

Merge policy

Do not merge until all CI checks are green.