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_test.go

@@ -344,7 +344,7 @@ func TestSceneBlocks(t *testing.T) {
 	defer g.Close()
 
 	blocks := []MemoryBlock{
-		{ID: "blk_a", Modality: "image", Text: "老大发的排班表截图", PayloadDigest: "aaaa1111bbbb2222", Scene: "chan:qq"},
+		{ID: "blk_a", Modality: "image", Text: "老大发的排班表截图", Data: []byte("png-bytes"), Scene: "chan:qq"},
 		{ID: "blk_b", Modality: "text", Text: "无关场面的转写"},
 	}
 	if err := g.PutMemoryBlocks(blocks); err != nil {
@@ -375,7 +375,7 @@ func TestSceneBlocks(t *testing.T) {
 
 	// 无场景重写不得抹掉已挂的场景（静默降级防护）
 	if err := g.PutMemoryBlocks([]MemoryBlock{
-		{ID: "blk_a", Modality: "image", Text: "重新描述", PayloadDigest: "aaaa1111bbbb2222"},
+		{ID: "blk_a", Modality: "image", Text: "重新描述", Data: []byte("png-bytes")},
 	}); err != nil {
 		t.Fatalf("重写块: %v", err)
 	}
@@ -468,7 +468,7 @@ func TestFormatContextSceneBlocks(t *testing.T) {
 		t.Fatal(err)
 	}
 	if err := g.PutMemoryBlocks([]MemoryBlock{
-		{ID: "blk_x", Modality: "image", Text: "老大发的排班表截图", PayloadDigest: "cccc3333dddd4444", Scene: "chan:qq"},
+		{ID: "blk_x", Modality: "image", Text: "老大发的排班表截图", Data: []byte("png-bytes"), Scene: "chan:qq"},
 	}); err != nil {
 		t.Fatal(err)
 	}