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

helpers.go

@@ -0,0 +1,73 @@
+package httpclient
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"io"
+	"net/http"
+
+	"code.nochebuena.dev/einherjar/core/xerrors"
+)
+
+// DoJSON executes req and decodes the JSON response body into T.
+// Returns a xerrors-typed error for HTTP 4xx/5xx responses.
+func DoJSON[T any](ctx context.Context, client Provider, req *http.Request) (*T, error) {
+	req = req.WithContext(ctx)
+	resp, err := client.Do(req)
+	if err != nil {
+		return nil, err
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode >= 400 {
+		return nil, MapStatusToError(resp.StatusCode, "external API error")
+	}
+
+	body, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return nil, xerrors.New(xerrors.ErrInternal, "failed to read response body").WithError(err)
+	}
+
+	var data T
+	if err := json.Unmarshal(body, &data); err != nil {
+		return nil, xerrors.New(xerrors.ErrInternal, "failed to decode JSON response").WithError(err)
+	}
+
+	return &data, nil
+}
+
+// DoJSONRequest marshals body as JSON, sends it with the given method to rawURL,
+// and decodes the response into Resp. For requests without a body, use DoJSON instead.
+func DoJSONRequest[Req, Resp any](ctx context.Context, client Provider, method, rawURL string, body Req) (*Resp, error) {
+	data, err := json.Marshal(body)
+	if err != nil {
+		return nil, xerrors.New(xerrors.ErrInternal, "failed to encode request body").WithError(err)
+	}
+	req, err := http.NewRequestWithContext(ctx, method, rawURL, bytes.NewReader(data))
+	if err != nil {
+		return nil, xerrors.New(xerrors.ErrInternal, "failed to create request").WithError(err)
+	}
+	req.Header.Set("Content-Type", "application/json")
+	return DoJSON[Resp](ctx, client, req)
+}
+
+// MapStatusToError maps an HTTP status code to the matching xerrors type.
+func MapStatusToError(code int, msg string) error {
+	switch code {
+	case http.StatusNotFound:
+		return xerrors.New(xerrors.ErrNotFound, msg)
+	case http.StatusBadRequest:
+		return xerrors.New(xerrors.ErrInvalidInput, msg)
+	case http.StatusUnauthorized:
+		return xerrors.New(xerrors.ErrUnauthorized, msg)
+	case http.StatusForbidden:
+		return xerrors.New(xerrors.ErrPermissionDenied, msg)
+	case http.StatusConflict:
+		return xerrors.New(xerrors.ErrAlreadyExists, msg)
+	case http.StatusTooManyRequests:
+		return xerrors.New(xerrors.ErrUnavailable, msg)
+	default:
+		return xerrors.New(xerrors.ErrInternal, msg)
+	}
+}