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/plugins/webui/handler_graph_pulse_test.go

@@ -44,7 +44,7 @@ func (f *fakeGraphMemory) BlocksChangedSince(since time.Time, limit int) ([]sdk.
 		}
 		out = append(out, sdk.MemoryBlockView{
 			ID: b.ID, Modality: string(b.Modality), Text: b.Text,
-			PayloadDigest: b.PayloadDigest, MIME: b.MIME,
+			HasData: len(b.Data) > 0, MIME: b.MIME,
 			Source: b.Source, Tool: b.Tool, Scene: b.Scene,
 			CreatedAt: b.CreatedAt, UpdatedAt: b.UpdatedAt,
 		})
@@ -74,10 +74,10 @@ func graphFixture() map[string]interface{} {
 		//   修复前 pulse 读的是它，而它永远落在窗口外 ⇒ 恒返回空。
 		"memory_blocks": []memory.MemoryBlock{
 			{ID: "b_old", Modality: memory.BlockModality("text"), Text: "老块",
-				PayloadDigest: "d0", Vector: []float64{0.1, 0.2, 0.3},
+				Vector: []float64{0.1, 0.2, 0.3},
 				CreatedAt: old, UpdatedAt: now.Add(-8 * time.Hour)},
 			{ID: "b_new", Modality: memory.BlockModality("text"), Text: "刚学到的东西",
-				PayloadDigest: "d1", Vector: []float64{0.1, 0.2, 0.3},
+				Vector: []float64{0.1, 0.2, 0.3},
 				CreatedAt: now, UpdatedAt: now},
 		},
 	}
@@ -226,7 +226,7 @@ func TestGraphPulse_IgnoresFrozenLegacyNodes(t *testing.T) {
 		// 块表：有一条刚更新（活图谱的真实形态）
 		"memory_blocks": []memory.MemoryBlock{
 			{ID: "b_live", Modality: memory.BlockModality("text"), Text: "刚写入的块",
-				PayloadDigest: "dx", CreatedAt: now, UpdatedAt: now},
+				CreatedAt: now, UpdatedAt: now},
 		},
 	}