Reviewer caught two small gaps after Task 3:
- Effect A's smooth-scroll path on new messages is intentionally NOT
instrumented with the sentinel (the animation lands asynchronously
across frames; no intermediate scrollTop is worth pinning to). The
doc now records this so the reader's intuition matches the code.
- The sentinel-branch comment in MessageList.tsx pointed at "spec §2",
which is the planning doc rather than the durable subsystem spec.
Pointed at docs/systems/message-list.md instead.
Jannis Braun