docs(plan): sync Task 13 attach-snapshot with double-render fix

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

DOCS/superpowers/plans/2026-06-09-spacesh-m0-m1.md

@@ -2702,13 +2702,15 @@ pub fn spawn_surface(
                         }
                         Some(SurfaceMsg::Attach { reply }) => { let _ = reply.send(bcast.subscribe()); }
                         Some(SurfaceMsg::AttachSnapshot { reply }) => {
-                            // Flush pending into the grid first so the snapshot is current,
+                            // Subscribe + snapshot ONLY — do not touch `pending`.
-                            // but DO NOT broadcast here; subscribe before any further output.
+                            // This arm is atomic within the single actor (no await, no flush
-                            if !pending.is_empty() {
+                            // can interleave), so subscribing before snapshotting guarantees the
-                                grid.feed(&pending);
+                            // new receiver gets exactly the output emitted AFTER this snapshot.
-                                let _ = bcast.send(std::mem::take(&mut pending));
+                            // Any accumulated `pending` (not yet fed to the grid) is left alone:
-                                flush_deadline = None;
+                            // the normal 6ms/16KiB flush path delivers it to ALL subscribers —
-                            }
+                            // including this new one — exactly once, and it is NOT in the snapshot.
+                            // No gap, no double-render. (Broadcasting pending here would re-send
+                            // already-snapshotted bytes to the new client via the bridge path.)
                             let sub = bcast.subscribe();
                             let snap = snapshot_ansi(&grid);
                             let _ = reply.send((snap, sub));