fix: fifth review round (durable mount failures, {#key} hydration slots)

cursor[bot]:
- variant_mount_failed now sets the session's pendingEvent (without
  clobbering a still-pending generate), so a helper restart replays it
  onto /poll and a repair --reply resolves instead of returning
  unknown_poll_reply_id. live-resume's next action names the real event
  id instead of a literal EVENT_ID placeholder.
- Contract v2 text hydration strips {#key} DELIMITERS from the zip
  source (content stays; it always renders), so key blocks can no longer
  shift expression slots against the live DOM.

This work was produced with AI assistance (Claude Code).

Co-Authored-By: Claude Code <noreply@anthropic.com>

skill/scripts/live/session-store.mjs

@@ -410,6 +410,13 @@ function applyEvent(snapshot, entry) {
         },
       ].slice(-MOUNT_FAILURE_HISTORY);
       next.renderState = deriveRenderState(next);
+      // The failure needs an agent reply, so it must survive a helper
+      // restart the same way a generate does. Never clobber a still-pending
+      // generate: a progressive publish can fail an early mount while the
+      // generate event itself is still leased.
+      if (!next.pendingEvent) {
+        next.pendingEvent = toPendingEvent(event);
+      }
       break;
     }
     case 'checkpoint':