refactor(memory)!: 块 union 化，彻底淘汰 CAS 第一阶段（生产代码）

块自带负载（MemoryBlock.Data），删 PayloadDigest；检索纯向量，媒体随块
同生灭。生产代码全绿；CAS 专属测试将在下一提交随判据重写。

- memory: Data BLOB + migration9；blockColumns 不含 data（召回只比向量）；
  MediaBlockID 内容派生 ID；BlockByIDPrefix/BlockData 按需加载
- sdk: memory_impl 去 media.Store；MemoryBlockView PayloadDigest→HasData
- core: mediaref/graphmedia/crossmodal/medialoop 全部改块语义；
  captureBlockMedia 直接产块；resolveMediaRefs 按块 ID 解析
- knowledge: 媒体字节随条目目录自带（File+Data），删 mediaGet/mediaPut
- webui: graphBlockView/pulse payload_digest→has_data
- bootstrap/registry: 去 MediaStore 接线（SetMediaStore 空实现，待⑥删包）

internal/memory/graph_test.go

@@ -520,7 +520,7 @@ func TestMemoryBlocksAreFirstClassGraphNodes(t *testing.T) {
 
 	image := MemoryBlock{
 		ID: "block_image_1", Modality: BlockImage,
-		PayloadDigest: "0123456789abcdef", MIME: "image/png", Size: 1234,
+		Data: []byte("fake-png"), MIME: "image/png", Size: 1234,
 		Width: 768, Height: 512, Vector: []float64{0.1, 0.2, 0.3},
 		Fingerprint: "qwen:test", Source: "qq", Tool: "upload",
 	}
@@ -545,7 +545,7 @@ func TestMemoryBlocksAreFirstClassGraphNodes(t *testing.T) {
 			gotImage = &blocks[i]
 		}
 	}
-	if gotImage == nil || gotImage.Modality != BlockImage || gotImage.PayloadDigest != image.PayloadDigest || gotImage.Fingerprint != image.Fingerprint || len(gotImage.Vector) != 3 {
+	if gotImage == nil || gotImage.Modality != BlockImage || string(gotImage.Data) != string(image.Data) || gotImage.Fingerprint != image.Fingerprint || len(gotImage.Vector) != 3 {
 		t.Fatalf("image block not round-tripped: %+v", gotImage)
 	}