Code Review: #10

Decision: PASS

Findings

• [NIT] internal/relay/healthz_test.go:32 — len(body) >= 200 is checked against a body of ~85 bytes (version="test-version", counts=0). The assertion is correct and useful as a regression guard, but it doesn't exercise anything close to the spec's worst-case (~135 bytes). A second assertion with high-cardinality counts would tighten the budget — non-blocking; the present check still catches a structural regression.
• [NIT] internal/relay/healthz.go:36 — request param r is unused (handler ignores method/headers/body by design). Idiomatic Go keeps it named in handler signatures, so leaving it as r is the right call; flagging only because a future linter pass might suggest _.

Summary

The diff matches the architect's spec exactly: new internal/relay/healthz.go with unexported healthzResponse (field order pinned, JSON tags match the AC), NewHealthzHandler factory returning http.Handler, single Counts() call released before any I/O, Cache-Control: no-store set, time.Since floored at zero as documented defence-in-depth. main.go captures startedAt after the --version early-return (correct semantics — "began serving" rather than "binary started") and constructs the registry once so #4/#5/#16 won't have to refactor main.

Doc-comment on NewHealthzHandler names the unauthenticated-by-design intent and the concurrency contract (no per-request state, no goroutines, RLock released before I/O). The discarded json.Marshal error is documented inline; the marshal-then-write pattern is correctly justified for response atomicity.

Tests cover all 9 AC bullets across two cases. TestHealthz_ResponseShape asserts status code, both response headers, body-size budget, typed decode of all fields, and a separate map[string]json.RawMessage decode that would catch a Go-field rename that broke the wire contract — nice belt-and-suspenders. TestHealthz_TracksRegistryState correctly reuses the existing in-package fakeConn helper without re-export.

Security-sensitive label honoured: the architect's spec contains a ## Security review section with categories walked (trust boundaries, info disclosure, probe amplification, cache poisoning, races, header injection, method confusion, timing channels, supply chain) and a PASS verdict. Diff implementation matches every claim made in that review (no caller input reaches the response, headers are constant strings, aggregate counts only via Counts(), no new deps).

go vet ./... clean. go test -race ./internal/relay/... passes. CI test job green; security job (gosec/govulncheck) was still running at review time but there are no new dependencies and no caller-controlled string handling, so a clean result is the expected outcome.