Keep authority posture optional in NetEngine specs

[aidankhogg]

Motivation

• Preserve backwards compatibility so existing example YAMLs continue to parse without requiring a new top-level authorities section.
• Allow introducing authority posture as an importable domain object and opt-in overrides while deriving sensible defaults from existing spec fields.
• Provide generation helpers that honor optional overrides but fall back to stable defaults for the MVP.

Description

• Add AuthorityPosture Pydantic model to netengine.spec.models to represent optional authority posture overrides.
• Add an optional authority: AuthorityPosture | None = None field to NetEngineSpec so old specs remain valid when omitted.
• Re-export AuthorityPosture from netengine.spec for importability alongside existing authority domain objects.
• Update world_manifest_from_spec in netengine/spec/authority.py to honor provided posture overrides while deriving defaults from existing top-level spec sections when authority is absent, and add tests exercising both behaviors.

Testing

• Ran pytest tests/test_authority_spec.py tests/test_spec_parsing.py -q and the tests passed.
• Ran pytest -q (full test suite) which completed successfully with 627 passed, 10 skipped.
• Loaded the example specs with load_spec for examples/minimal.yaml, examples/single-org.yaml, and examples/dev-sandbox.yaml and confirmed each parsed with authority is None.

---

Codex Task