doc(tui): record rebuild cost, replay fixture, and the A2 fold interaction

Review follow-ups on b9b2e593f, all documentation precision.

The redundancy note on `isCompactCheckpoint` leaned on a reading its call
site does not state: `index.ts` reaches it for surface-eligible non-append
events, which is the same set as replacements only because the marker is
mandatory. Say that instead.

The Agent Note now owns three facts it was leaving to a future reader.
`rebuildTranscript` materializes a component per append-origin event and
runs on mount, color-scheme change, and every reasoning toggle — work
compaction used to bound for exactly the long sessions it serves, so the
cost now tracks session length rather than the surface. `Consequences`
names `surface-replayed-compaction` as the durable evidence for the
live/replay equivalence claim, so the two fixtures that must move together
are findable from the Note rather than the PR thread. `Deferred` records
that a page can now carry a checkpoint whose `surfaceOp.start` fell out of
the window: pagination no longer cuts on the checkpoint's provenance
group, `FoldAdapter` pads with a non-surface sentinel, and `nodes()`
degrades to `degradedSeqs()` — which is already close to the transcript
projection A2 should build deliberately.

Also corrects the definite-assignment comment in the snapshot scenario:
the assertion rests on the awaited setup invoking `beforeMount`, not on
that call being synchronous.

packages/ui/tui/src/chat/helpers.ts

@@ -110,9 +110,11 @@ export function transcriptToolCallIds(session: Session): Set<string> {
  * `tool/result`, a regenerated `assistant/message`) rewrite one node for the
  * model and mark no boundary in the conversation.
  *
- * The replacement check is redundant at both current call sites, which already
- * reached a replacement: it keeps the exported predicate true to its name for a
- * third caller, rather than making that caller repeat the check.
+ * The replacement check is redundant at both current call sites, because a
+ * surface-eligible non-append event is a replacement: the marker is mandatory,
+ * so `Session.append` and the replay fold reject an event that carries none.
+ * The check keeps the exported predicate true to its name for a third caller,
+ * rather than making that caller repeat it.
  * @param event - event to test.
  * @returns true when the event compacted a surface range.
  */

packages/ui/tui/tests/tui.snapshot.ts

@@ -749,8 +749,8 @@ describe('TUI terminal-state snapshots', () => {
     // real-clock millisecond tick between the fixture appends and the render
     // would flip `Tools 0.0s` in and out of the pinned header.
     const nowSpy = vi.spyOn(Date, 'now').mockReturnValue(COMPACTION_FIXTURE_TIME)
-    // beforeMount runs synchronously inside setupSnapshot, so the range the
-    // checkpoint replaces is assigned before the first await below.
+    // The awaited setup always invokes beforeMount, so the range the checkpoint
+    // replaces is assigned by the time the appends below need it.
     let compacted!: CompactionRange
     const harness = await setupSnapshot({
       tools: ADVANCED_CARD_TOOLS,