feat(session-query): expose projectSessions batch projection

Public SessionQueryService.projectSessions wraps the existing corpus
projectMany: one persistence listing, bounded persisted-inspect
concurrency, per-id failure isolation, and a synchronous projector over
a borrowed source with no replay validation or cloning.
readTitleSnapshots now routes through it; LogicalSessionSource and
LogicalProjectionResult are exported and documented.

docs/core-data-structures/session-query.zh.md

@@ -84,6 +84,25 @@ type SessionTitleObservationResult =
   }
 ```
 
+`projectSessions` 在同一实时优先语料库上批量执行任意同步折叠：每个 `LogicalSessionSource` 都是借用的原始日志——从不做回放验证，也从不克隆——仅在投影函数调用期间有效，因此一次批量摘要对每个持久化会话只需一次有界读取。每个 `LogicalProjectionResult` 按唯一请求 id 结算，其失败隔离与取消规则与批量标题读取一致。
+
+```ts type-equiv
+/** Borrowed source visible only during one synchronous batch projection. */
+interface LogicalSessionSource {
+  /** Header selected with `events`; callers must clone retained output. */
+  readonly header: SessionHeader
+  /** Raw events selected with `header`; valid only for the projection call. */
+  readonly events: readonly SessionEvent[]
+}
+```
+
+```ts type-equiv
+/** One source-projection result in a batch logical-corpus observation. */
+type LogicalProjectionResult<Value> =
+  | { sessionId: SessionId; status: 'fulfilled'; value: Value }
+  | { sessionId: SessionId; status: 'rejected'; reason: unknown }
+```
+
 ```ts type-equiv
 /** Lightweight metadata for one event within a logical session. */
 interface SessionEventRecord {