Field-registry health snapshot (read-only)

Deprecated: moved under /v1/fields/registry/*; use GET /v1/fields/registry/health. This path remains supported and serves the same handler. Tier + admission counts, singleton/weak-name rates, occurrence and resolution stats, transfer hit rate, and coherence/atomicity metrics for the tenant's field registry. The same view the in-app registry dashboard reads; no lifecycle or cleanup actions are exposed.