feat(httpclient): initial implementation — HTTP client with retry and circuit breaker (v1.0.0)

Introduces code.nochebuena.dev/einherjar/httpclient — the outbound HTTP client
starter for the Einherjar framework. Absorbs the httpclient package from micro-lib,
replacing fmt.Errorf wrapping with core/xerrors and adding generic JSON helpers.

Interfaces:
- Provider — Do(req *http.Request) (*http.Response, error)

Implementation:
- New(logger, cfg) Provider — configures net.Dialer + retry + circuit breaker
- NewWithDefaults(logger) Provider — convenience constructor with default config
- Retry: avast/retry-go; configurable MaxRetries and RetryDelay; retries on
  network errors and 5xx responses; logs each retry attempt at Warn level
- Circuit breaker: sony/gobreaker; opens after CBThreshold consecutive failures
  within CBTimeout window; returns ErrUnavailable when open
- DoJSON[T](ctx, client, req) (*T, error) — executes request, decodes JSON body
- DoJSONRequest[Req, Resp](ctx, client, method, rawURL, body) (*Resp, error) —
  marshals body, builds request, executes, decodes response
- MapStatusToError(code, msg) error — maps HTTP status codes to xerrors values

Config (EINHERJAR_HTTP_* env vars):
  Name(http), Timeout(30s), DialTimeout(5s),
  MaxRetries(3), RetryDelay(1s), CBThreshold(10), CBTimeout(1m)

- identifiable.go: package-level Module variable (observability.Identifiable) for version
  identification — httpclient is a stateless provider; not registered with the launcher

provider.go

@@ -0,0 +1,9 @@
+package httpclient
+
+import "net/http"
+
+// Provider executes HTTP requests with automatic retry and circuit breaking.
+// Inject Provider into services that make outbound HTTP calls; construct with New or NewWithDefaults.
+type Provider interface {
+	Do(req *http.Request) (*http.Response, error)
+}