Add ResolverPolicy model and derivation from BoundaryPolicy

[aidankhogg]

Motivation

• Provide a small, explicit model of resolver capabilities derived from boundary posture to centralize resolver-related reasoning.
• Offer a helper to convert existing BoundaryPolicy values into a minimal, well-documented set of resolver semantics for downstream consumers.

Description

• Add ResolverPolicy to netengine/spec/authority.py with fields local_root, upstream, mirror_table, peer_suffix_delegation, imported_trust_bundle, and notes to model resolver capabilities.
• Implement resolver_policy_from_boundary(policy: BoundaryPolicy) -> ResolverPolicy in netengine/spec/authority.py that maps BoundaryPolicy to minimum semantics for isolated, shadowed, mirrored, peered, and federated modes and records human-readable precedence notes.
• Export ResolverPolicy and resolver_policy_from_boundary via netengine/spec/__init__.py for package-level consumption.
• Add unit tests in tests/test_authority_spec.py that cover default ResolverPolicy values and derivation behavior for isolated, shadowed, mirrored, peered, and federated boundary configurations.

Testing

• Ran pytest tests/test_authority_spec.py and the new authority/resolver tests passed.
• Ran the full test suite with pytest and observed all tests passing in this environment (618 passed, 10 skipped).

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Codex Task