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/migrate_entities.go

@@ -487,7 +487,7 @@ func putBlockTx(tx *sql.Tx, b MemoryBlock) (bool, error) {
 	}
 
 	_, err := tx.Exec(`INSERT INTO memory_blocks
-		(id, modality, text_content, payload_digest, mime, size, width, height,
+		(id, modality, text_content, data, mime, size, width, height,
 		 vector, fingerprint, source, tool, scene, semantic_type, created_at, updated_at)
 		VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)
 		ON CONFLICT(id) DO UPDATE SET
@@ -499,7 +499,7 @@ func putBlockTx(tx *sql.Tx, b MemoryBlock) (bool, error) {
 			semantic_type = CASE WHEN excluded.semantic_type != ''
 				THEN excluded.semantic_type ELSE memory_blocks.semantic_type END,
 			`+tsUpdate,
-		b.ID, b.Modality, b.Text, b.PayloadDigest, b.MIME, b.Size, b.Width, b.Height,
+		b.ID, b.Modality, b.Text, b.Data, b.MIME, b.Size, b.Width, b.Height,
 		vectorJSON, b.Fingerprint, b.Source, b.Tool, b.Scene, b.SemanticType,
 		created, updated)
 	if err != nil {