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/scene.go

@@ -471,7 +471,7 @@ func (g *GraphDB) RecallByScene(scenes []string, limit int) (*SceneRecall, error
 		}
 	}
 
-	blockQuery := `SELECT b.id, b.modality, b.text_content, b.payload_digest, b.mime,
+	blockQuery := `SELECT b.id, b.modality, b.text_content, b.mime,
 			b.size, b.width, b.height, b.fingerprint, b.source, b.tool,
 			COALESCE(b.scene, ''), COALESCE(b.semantic_type, ''),
 			b.created_at, b.updated_at, MAX(sr.weight) AS w
@@ -491,7 +491,7 @@ func (g *GraphDB) RecallByScene(scenes []string, limit int) (*SceneRecall, error
 	for brows.Next() {
 		var b MemoryBlock
 		var w float64
-		if err := brows.Scan(&b.ID, &b.Modality, &b.Text, &b.PayloadDigest, &b.MIME,
+		if err := brows.Scan(&b.ID, &b.Modality, &b.Text, &b.MIME,
 			&b.Size, &b.Width, &b.Height, &b.Fingerprint, &b.Source, &b.Tool,
 			&b.Scene, &b.SemanticType, &b.CreatedAt, &b.UpdatedAt, &w); err != nil {
 			return nil, err