fix(tui,host): project the human transcript from append-origin events

The terminal and history pagination both treated the model-visible surface as
the human transcript. A landed compaction replacement therefore erased the
conversation it summarized — messages the reader had already seen — and a
model-only replacement copy consumed a page's `maxMessages` quota, which could
also split a compaction's provenance from the replacement citing it.

`dsh-session` now exports the marker split `isAppendSurfaceEvent` /
`isReplacementSurfaceEvent`. The terminal replays append-origin surface events,
keeps a shadowed step's tool cards paired through its append-origin assistant
message, and renders one dim marker where a compaction landed; the checkpoint is
recognized through the compaction seam's `isCompactCheckpointSource` contract,
not the shape of the replacement. `session.history` counts only append-origin
human messages. Everything model-facing keeps reading `session.surface`.

packages/core/session/src/index.ts

@@ -27,7 +27,7 @@ export { interruptedTurnClosers, TOOL_NOT_STARTED, TOOL_OUTCOME_UNKNOWN } from '
 export { decodeStorageRecord, packChunkRuns } from './chunk-rows.ts'
 export type { ChunkRow, StorageRecord } from './chunk-rows.ts'
 export type { SessionSurface, SurfaceFoldReplacement, SurfaceFoldResult } from './surface.ts'
-export { foldSurface, isSurfaceEvent, isSurfaceEligibleType } from './surface.ts'
+export { foldSurface, isAppendSurfaceEvent, isReplacementSurfaceEvent, isSurfaceEvent, isSurfaceEligibleType } from './surface.ts'
 export { canonicalHeader, foldRequestHeader, headerEquals } from './request-header.ts'
 
 /**

packages/core/session/src/surface.ts

@@ -37,6 +37,36 @@ export function isSurfaceEvent(event: SessionEvent): event is SurfaceEvent {
   return (event as SessionEvent<SurfaceEventType>).surfaceOp !== undefined
 }
 
+/**
+ * Narrow an event to an append-origin surface event: one that entered the
+ * surface at its own log position and was never itself a replacement copy.
+ *
+ * The model-visible surface deliberately shadows replaced ranges, so it is the
+ * wrong source for a human transcript — a landed replacement would erase
+ * conversation the user already saw. Append-origin events are that transcript's
+ * durable source material; replacement copies stay model-only.
+ * @param event - event to test.
+ * @returns true when the event appended to the surface tail.
+ */
+export function isAppendSurfaceEvent(
+  event: SessionEvent,
+): event is SurfaceEvent & { surfaceOp: 'append' } {
+  return isSurfaceEvent(event) && event.surfaceOp === 'append'
+}
+
+/**
+ * Narrow an event to a surface replacement: a node that shadowed an existing
+ * surface range instead of appending to the tail. The counterpart of
+ * {@link isAppendSurfaceEvent} over the two {@link SurfaceOp} variants.
+ * @param event - event to test.
+ * @returns true when the event replaced a surface range.
+ */
+export function isReplacementSurfaceEvent(
+  event: SessionEvent,
+): event is SurfaceEvent & { surfaceOp: { op: 'replace'; start: number; end: number } } {
+  return isSurfaceEvent(event) && event.surfaceOp !== 'append'
+}
+
 /** One replacement operation observed while folding a session surface. */
 export interface SurfaceFoldReplacement {
   /** Seq of the event that replaced the prior surface range. */