-`Logger` interface — duck-typed minimal logger (`Debug`, `Info`, `Warn`, `Error`, `WithContext`) satisfied by `logz.Logger` without importing logz
-`ComponentStatus` struct — JSON-serialisable per-component result with fields `status string`, `latency string` (omitempty), and `error string` (omitempty)
-`Response` struct — JSON-serialisable overall response with fields `status string` and `components map[string]ComponentStatus`
-`NewHandler(logger Logger, checks ...Checkable) http.Handler` — constructs an `http.Handler` that runs all registered checks concurrently, collects results via a buffered channel, and writes a JSON `Response`
- Parallel check execution: each `Checkable.HealthCheck` call runs in its own goroutine; a buffered channel sized to the check count prevents goroutine leaks
- Per-request 5-second deadline derived from `context.WithTimeout(r.Context(), 5*time.Second)` and propagated to all check goroutines
- Overall status aggregation: `UP` if all checks pass; `DEGRADED` (HTTP 200) if at least one degraded component fails and no critical component fails; `DOWN` (HTTP 503) if any critical component fails
- Check latency measurement: each goroutine records `time.Since(start)` and includes it as a string in `ComponentStatus`
-`Content-Type: application/json` response header set on every response
### Design Notes
- All checks run in parallel goroutines and report through a buffered channel; the buffer is sized exactly to the number of registered checks at construction time, so the handler is guaranteed to drain all results without blocking even if it returns early.
- The two-level criticality model (`LevelCritical` / `LevelDegraded`) gives orchestrators and load balancers a clean binary HTTP signal (200 vs 503) while still surfacing partial degradation in the JSON body for monitoring systems.
- The `Logger` and `Checkable` interfaces are defined entirely within this package using duck typing — no micro-lib module is imported, keeping `health` a pure stdlib package (Tier 0/1) that infra packages can satisfy without a circular dependency.
