round 1: project the web transcript from append-origin events

Replace the surface-ordered fold with a log-ordered human transcript:
append-origin surface events at their own log positions plus one marker per
landed compaction checkpoint. Command folding, the tool-call index, and the
rev-keyed memo carry over unchanged.

Removes foldDegraded, the padding sentinels, baseSeq, and degradedSeqs() --
they existed only to satisfy the core fold's seq === index assertion. That
also closes the pagination hole A1 exposed: a page can carry a checkpoint
whose shadowed range fell outside the window, and nothing resolves
surfaceOp.start anymore.

packages/client/ui-trajectory/src/client/layout.ts

@@ -110,8 +110,8 @@ export function deriveTrajectoryLayout(input: TrajectoryLayoutInput): readonly T
       lastAssistantTurn = node.turn
       continue
     }
-    if (node.kind === 'context') {
-      // No trajectory cell, but the surface still advances the duration cursor.
+    if (node.kind === 'context' || node.kind === 'compaction') {
+      // No trajectory cell, but transcript metadata still advances the duration cursor.
       prevAbsTime = finiteTime(node.time) ?? prevAbsTime
       continue
     }

packages/client/ui-trajectory/src/client/spans.ts

@@ -45,7 +45,7 @@ export interface SpanStats {
  * Fold snapshot nodes into per-turn spans. Only assistant nodes carry a turn
  * number; user/steering/context/tool nodes attach to the turn last seen in
  * sequence order (turn 0 collects the pre-assistant prologue).
- * @param nodes - snapshot nodes in surface order.
+ * @param nodes - snapshot nodes in human transcript order.
  * @returns spans ordered by first appearance.
  */
 export function deriveSpans(nodes: ConversationSnapshot['nodes']): readonly TurnSpan[] {