Implement labeler identity stage with per-check diagnostics
Adds the identity stage that performs DID document resolution and labeler
record validation. Implements 9 named checks with per-check diagnostics:
target_resolved, did_document_fetched, labeler_service_present,
labeler_endpoint_is_https, labeler_endpoint_matches_flag,
signing_key_present, pds_endpoint_present, labeler_record_fetched,
labeler_record_policies_nonempty.
Each failing check carries a diagnostic with NamedSource and span
highlighting the relevant JSON field. Network errors are distinguished
from specification violations. Facts are populated only when all checks
pass, blocking later stages if unavailable.
Verifies test-labeler.AC2.1 through AC2.8 and AC1.4 (endpoint/DID cross-check).
Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>