fix(trajectory): preserve state across history prepends

docs/core-data-structures/session.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write docs/core-data-structures/session.md
-session.md: 6369c956df03c0d786db696c208b000300d5bfbc
-session.zh.md: c39382b7e6c9b14f91c311cc80526a6fd8898e4c
+session.md: b2d4e093a019df5c43f705808b964b9cd7606834
+session.zh.md: ce37052244c69ea6c10a479ca9e92ed012e93a72

docs/core-data-structures/session.md

@@ -311,6 +311,8 @@ The same provenance distinction applies here: only `assistant/message` may carry
 
 `Session.surface` returns the session's stable `SessionSurface` view. The same incremental manager validates append candidates before commit and advances this projection from committed events; callers can observe membership and replacement generation but cannot invoke validation.
 
+`SurfaceManager(log, baseSeq?)` can instead fold a contiguous loaded window whose first event has the absolute sequence `baseSeq`. Every event remains contiguous in that absolute sequence space, and a replacement that crosses the window head fails because its declared range is absent.
+
 ```ts type-equiv
 /** Readonly live projection of the message-producing session events. */
 interface SessionSurface {

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

@@ -313,6 +313,8 @@ interface SurfaceIntent {
 
 `Session.surface` 返回会话稳定的 `SessionSurface` 视图。同一个增量管理器在提交前校验追加候选事件，并根据已提交事件推进该投影；调用方可以观察成员关系和替换代次，但不能调用校验。
 
+`SurfaceManager(log, baseSeq?)` 也可以折叠一个连续的已加载窗口，其第一个事件的绝对序号为 `baseSeq`。每个事件在该绝对序号空间中仍保持连续；如果替换跨过窗口头部，由于其声明的范围并不存在，该替换会失败。
+
 ```ts type-equiv
 /** Readonly live projection of the message-producing session events. */
 interface SessionSurface {