fix(session-export): distinguish unsupported raw artifacts

SessionPersistence.readRaw previously used undefined for two unrelated states: a supported backend could not find the requested session, or the backend had no per-session artifact concept at all. The export endpoint consequently reported an existing SQLite-backed session as HTTP 404, which falsely diagnosed storage capability as session absence.

Make raw-artifact support an explicit backend capability. Unsupported backends now fail their inherited readRaw path loudly and the host answers 501 before reading, while undefined retains the single meaning of an absent artifact on a supporting backend. First-party backends, test providers, generated API catalogs, bilingual persistence docs, and export error contracts now state that distinction; focused tests cover both the 501 and the inherited rejection.

packages/session/session-persistence/README.zh.md

@@ -11,6 +11,8 @@
 | 方法 | 约定 |
 |---|---|
 | `locate(meta): SessionLocation \| undefined` | 在不执行 I/O 或实体化的情况下解析绝对的每会话产物目标。没有独立本地产物的后端返回 `undefined`。 |
+| `supportsRawArtifacts: boolean` | 明确说明该后端是否为每个会话暴露一份逐字工件。Consumer 在调用 `readRaw` 前检查此能力；`false` 并不表示会话缺失。 |
+| `readRaw(id, signal?): Promise<SessionRawArtifact \| undefined>` | 读取受支持后端自身的逐字工件文本；只解码物理编码，绝不从事件重建。`undefined` 仅表示所请求工件缺失；不支持的后端会拒绝。 |
 | `create(meta): Promise<void>` | 注册新会话元数据。可以将物理写入延迟到第一次 `append`（延迟实体化）。 |
 | `append(id, events): Promise<void>` | 持久保存一个批次。仅追加；任何修复后，第一个事件 `seq` == 已存储 next-seq；非 JSON 可序列化数据会被拒绝，并命名违规类型。 |
 | `prepare(id, signal?): Promise<SessionPreparation>` | 预留恢复所使用的那个未发布 Session。协调器会尽可能复用之前的检查结果、提交待处理恢复，并在 dispose 时将未发布 reservation 释放回有界缓存。 |