fix(session-query): address review round 1

docs/core-data-structures/session.md

@@ -196,6 +196,26 @@ export interface SurfaceNode {
 }
 ```
 
+### `SurfaceFoldReplacement` and `SurfaceFoldResult` — a complete surface replay
+
+`foldSurface(events)` returns detached current nodes together with the actual node seqs shadowed by each declared replacement range. `SurfaceManager` uses the same transition functions for its incremental cache.
+
+```ts type-equiv
+export interface SurfaceFoldReplacement {
+  seq: number
+  start: number
+  end: number
+  shadowedSeqs: number[]
+}
+```
+
+```ts type-equiv
+export interface SurfaceFoldResult {
+  nodes: SurfaceNode[]
+  replacements: SurfaceFoldReplacement[]
+}
+```
+
 ## Derived history: `deriveMessages()` and `deriveEventMessage()`
 
 `Session.deriveMessages()` projects the event log into the `Message[]` the model sees — cached (each surface node projected once, when first seen; a surface rewrite rebuilds) and frozen (a fresh array per call over shared, deep-frozen messages, so mutating logged history through a projection is unrepresentable). `deriveEventMessage(event)` is the per-node pure function the fold applies — public so external reconstructors and the dev invariant project a log prefix with exactly the same rules and cannot disagree with the cache. The projection rules: