CAT-PB: buildstep.PushBlob is removed, its last users write through the seats

No build module writes into a store any more: mesh-build-go's store test
plays the holder with its own two-request upload, as mesh-build-node's does.

Claude-Session: https://claude.ai/code/session_01JG73KJNrH5EgFXMKKYSrig

modules/mesh-build-go/cmd/mesh-build-go/store_integration_test.go

@@ -3,13 +3,16 @@
 package main
 
 import (
+	"bytes"
 	"context"
 	"crypto/sha256"
 	"encoding/hex"
 	"encoding/json"
+	"fmt"
 	"io"
 	"net/http"
 	"net/http/httptest"
+	"net/url"
 	"os"
 	"os/exec"
 	"path/filepath"
@@ -23,8 +26,6 @@ import (
 	registry "git.novox.be/novox/mesh-contracts/go/seats/containerimageregistry"
 	"git.novox.be/novox/mesh-sdk/go/catalogue"
 	"git.novox.be/novox/mesh-sdk/go/delivery"
-
-	"git.novox.be/novox/mesh-catalog/modules/mesh-build-go/buildstep"
 )
 
 // realStore starts the artifact store's own program (the distribution registry, modules/distribution) inside this
@@ -53,7 +54,7 @@ func realStore(t *testing.T, m *Module) string {
 		defer s.mu.Unlock()
 		for k, b := range s.blobs {
 			name, digest, _ := strings.Cut(k, "@")
-			if err := buildstep.PushBlob(context.Background(), http.DefaultClient, addr, name, digest, b); err != nil {
+			if err := uploadBlob(srv.URL, name, digest, b); err != nil {
 				return nil, err
 			}
 			delete(s.blobs, k)
@@ -63,6 +64,36 @@ func realStore(t *testing.T, m *Module) string {
 	return addr
 }
 
+// uploadBlob is the holder's own write into its store: an upload opened, then its bytes put by their digest (the
+// distribution protocol's two requests), as distribution stores a part it took whole.
+func uploadBlob(base, name, digest string, body []byte) error {
+	resp, err := http.Post(base+"/v2/"+name+"/blobs/uploads/", "", nil)
+	if err != nil {
+		return err
+	}
+	resp.Body.Close()
+	loc, err := url.Parse(resp.Header.Get("Location"))
+	if err != nil || resp.StatusCode != http.StatusAccepted {
+		return fmt.Errorf("the store opened no upload of %s: %s %v", digest, resp.Status, err)
+	}
+	b, _ := url.Parse(base)
+	u := b.ResolveReference(loc)
+	q := u.Query()
+	q.Set("digest", digest)
+	u.RawQuery = q.Encode()
+	req, _ := http.NewRequest(http.MethodPut, u.String(), bytes.NewReader(body))
+	req.Header.Set("Content-Type", "application/octet-stream")
+	put, err := http.DefaultClient.Do(req)
+	if err != nil {
+		return err
+	}
+	put.Body.Close()
+	if put.StatusCode != http.StatusCreated {
+		return fmt.Errorf("the store did not take %s: %s", digest, put.Status)
+	}
+	return nil
+}
+
 // resolve reads a done artifact's reference from the store as a node-engine does (catalogue.Routed): the seat's form
 // oci-artifact-registry/<module>/<artifact>@<digest> is <store>/v2/<module>/<artifact>/blobs/<digest> (ADR 0605 §9).
 func resolve(t *testing.T, addr string, a delivery.BuiltArtifact) []byte {