#207 fix(gcp): expose read-API /healthz and accept deny-502 in native proof

Summary

• Native umbrella edge routed every unmatched history-host path to the selector-less deny Service (Google answers 502), but verify-public.sh requires 2xx on history.<base>/healthz. Adds an Exact /healthz route to the native read edge — the same probe the GCE health check already uses.
• public.history_internal_denied expected 404/403 but the deny backend answers 502; sibling deny cases already accept 502 as edge_denied. Made this case consistent. The isolation property (internal paths never reach the read API) is unchanged.

Validation

• helm template renders the new Exact /healthz rule.
• Live edge confirmed: /healthz → 502 before the change, public.history_internal_denied was the only verifier gap remaining (verifier was 15/17, the other being this case and the history health URL).

Generated with Devin

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Summary by cubic

Fixes two live-proof failures on the native GKE edge so verify-public.sh passes.

• Adds an Exact /healthz route to the native read edge; previously the umbrella routed it to the selector-less deny Service and Google answered 502.
• Accepts 502 in public.history_internal_denied since the deny backend answers 502; internal paths still never reach the read API, so the isolation property is unchanged.
• Bumps agent-relay to 0.6.52, registers it as the current release row, and marks 0.6.51 as planned.