Add foundational Authority spec model and enums

[aidankhogg]

Motivation

• Introduce a foundational authority primitive to represent who controls spec surfaces and where they originate, enabling later wiring of registry/identity/trust interactions in the spec domain.

Description

• Add netengine/spec/authority.py defining AuthorityKind, AuthorityScope, and AuthoritySource enums with the requested values and an immutable Pydantic Authority model (fields: id, kind, scope, operator, controls, description: str | None, source defaulting to AuthoritySource.LOCAL).
• Export Authority, AuthorityKind, AuthorityScope, and AuthoritySource from netengine/spec/__init__.py for package-level access.
• Add tests/test_authority_spec.py with unit tests validating default source, string-to-enum coercion, and model immutability.

Testing

• Ran the new unit tests with poetry run pytest tests/test_authority_spec.py and they passed.
• Ran the spec parsing tests poetry run pytest tests/test_spec_parsing.py tests/test_authority_spec.py and they passed.
• Ran the full test suite with poetry run pytest and all tests passed (existing tests + new ones: passed, with some tests skipped as in CI).

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Codex Task