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

doc.go

@@ -0,0 +1,44 @@
+// Package httpclient provides a resilient HTTP client with automatic retry,
+// circuit breaking, request-ID propagation, and typed JSON helpers.
+//
+// # Basic Usage
+//
+//	client := httpclient.NewWithDefaults(logger)
+//	resp, err := client.Do(req)
+//
+// # Typed JSON Helpers
+//
+// DoJSON decodes the response body into T without needing a manual http.Request:
+//
+//	type UserResp struct{ ID, Name string }
+//	req, _ := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
+//	user, err := httpclient.DoJSON[UserResp](ctx, client, req)
+//
+// DoJSONRequest marshals a request body, sends it, and decodes the response:
+//
+//	result, err := httpclient.DoJSONRequest[CreateReq, CreateResp](
+//	    ctx, client, http.MethodPost, url, createReq)
+//
+// # Request ID Propagation
+//
+// If the context carries a request ID (set by core/logz.WithRequestID or the
+// web/mw.RequestID middleware), it is forwarded as X-Request-ID on every
+// outbound attempt, including retries.
+//
+// # Resilience
+//
+// The retry loop (avast/retry-go) wraps individual HTTP attempts. The circuit
+// breaker (sony/gobreaker) wraps the entire retry sequence, so the breaker
+// opens after CBThreshold fully-exhausted retry sequences fail — not after
+// CBThreshold individual HTTP errors.
+//
+// # Configuration
+//
+//	EINHERJAR_HTTP_CLIENT_NAME  — circuit breaker label; default "http"
+//	EINHERJAR_HTTP_TIMEOUT      — total request timeout; default 30s
+//	EINHERJAR_HTTP_DIAL_TIMEOUT — TCP dial timeout; default 5s
+//	EINHERJAR_HTTP_MAX_RETRIES  — attempts per request; default 3
+//	EINHERJAR_HTTP_RETRY_DELAY  — base delay between retries; default 1s
+//	EINHERJAR_HTTP_CB_THRESHOLD — consecutive failures to open breaker; default 10
+//	EINHERJAR_HTTP_CB_TIMEOUT   — breaker half-open probe interval; default 1m
+package httpclient