Add WorldManifest model and world_manifest_from_spec helper

[aidankhogg]

Motivation

• Provide a compact authority/spec-domain manifest that captures world identity, lifecycle, authority bindings, gateway posture, and cross-world trust in a single model.
• Seed manifest identity from the parsed spec so tooling can refer to a stable world_id/world_name even when no stronger world-id is present in the spec (spec.metadata.name is used as the initial value).

Description

• Add WorldManifest Pydantic model in netengine.spec.authority with fields: world_id, world_name, lifecycle, authority_model, authorities, dns_root_authority, ca_trust_authority, platform_identity_issuer, inworld_identity_issuer, world_registry_authority, domain_registry_authority, numbering_authority, transit_boundary_authority, real_internet_posture, cross_world_posture, exported_authority_metadata, importable_authority_metadata, and trust_bundles.
• Add _trust_bundles_from_spec helper to derive TrustBundle entries from spec.gateway_portal.cross_world.peers and add world_manifest_from_spec(spec: NetEngineSpec) -> WorldManifest which seeds both world_id and world_name from spec.metadata.name, wires default authority IDs via default_authorities_for_spec, copies gateway posture, and attaches derived trust bundles.
• Export WorldManifest and world_manifest_from_spec from netengine.spec by updating netengine/spec/__init__.py.
• Add unit tests in tests/test_authority_spec.py covering the manifest defaults and cross-world trust bundle derivation.

Testing

• Ran pytest tests/test_authority_spec.py -q and received all tests passing (24 passed).
• Ran python -m compileall netengine/spec/authority.py netengine/spec/__init__.py tests/test_authority_spec.py which completed successfully.

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Codex Task