docs: document SessionRawArtifact and fix backpressure claims

The persistence subsystem page gains the SessionRawArtifact section (with a
type-equiv manifest entry and zh counterpart), and the README and Agent Note
describe the drain-based backpressure instead of claiming the archive never
accumulates.

docs/subsystems/persistence.zh.md

@@ -118,6 +118,22 @@ interface CreateSessionOptions {
 
 因此，回放/fork 的调用方式为 `ctx.sessions.create(id, { seed: seedEvents })`；将一个*持久化*会话恢复为活跃 agent 的调用方式为 `ctx.agents.resume({ resumeSessionId })`。
 
+## `SessionRawArtifact`——逐字存储工件文本
+
+后端为单个会话自持的工件文本，与其持久化写入的字节逐字一致（按物理编码解码）。`readRaw` 返回它而不从解析后事件重建，因此后端特定的序列化（chunk 打包、键序、换行）得以保留；没有每会话工件的后端（如 SQLite）继承 `undefined` 默认。
+
+```ts type-equiv
+/** A backend's own raw artifact text for one session, verbatim. */
+interface SessionRawArtifact {
+  /** The session header parsed from the artifact's own first line. */
+  readonly meta: SessionHeader
+  /** The artifact's base filename on disk, without any physical encoding suffix. */
+  readonly filename: string
+  /** The artifact's full text content, decoded from the backend's physical encoding. */
+  readonly content: string
+}
+```
+
 ## 准备与恢复所有权
 
 `SessionStore.prepare()` 接收普通创建选项，或通过 `RestoredSessionOptions` 转移所有权的新鲜持久化对象图。恢复分支会直接验证并冻结转移来的 header 与事件，因此调用方不得保留可变别名。`SessionPreparation` 随后持有该精确的未发布 Session，直至发布或回滚；dispose 是同步且幂等的。持久化检查只暴露 `SessionInspection`，即从同一个已准备 Session 借用的不可变逻辑视图。
@@ -250,7 +266,7 @@ abstract locate(meta: SessionHeader): SessionLocation | undefined
  * @returns the raw artifact plus its parsed header, or `undefined` when the
  * session is absent or the backend owns no per-session artifact.
  */
-readRaw(_id: SessionId, signal?: AbortSignal): Promise<SessionRawArtifact | undefined>
+async readRaw(_id: SessionId, signal?: AbortSignal): Promise<SessionRawArtifact | undefined>
 
 /**
  * Register a new session's metadata. A backend MAY defer the physical write