docs(message-list): note smooth-scroll exclusion; point sentinel comment at subsystem doc

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.

docs/systems/message-list.md

@@ -15,7 +15,7 @@ The chat message list (`packages/web/src/components/chat/MessageList.tsx`) is re
 
 Three effects cooperate. Their ordering is established by the 2026-03-25 race-fix and the 2026-04-25 sentinel addendum.
 
-**Effect A — initial snap / restore.** Runs once when `messages.length` transitions from 0 to N for a channel. Reads `chatStore.scrollPositions.get(channelId)`. If a saved anchor exists, scrolls that message into view and computes the resulting `isAtBottomRef` from actual distance. Otherwise, sets `container.scrollTop = container.scrollHeight`, captures the post-clamp value into `lastProgrammaticBottomScrollRef`, and sets `isAtBottomRef.current = true`. On subsequent message arrivals (`messages.length > prev`), if `isAtBottomRef.current`, smooth-scrolls via `bottomRef.scrollIntoView({ behavior: 'smooth' })`.
+**Effect A — initial snap / restore.** Runs once when `messages.length` transitions from 0 to N for a channel. Reads `chatStore.scrollPositions.get(channelId)`. If a saved anchor exists, scrolls that message into view and computes the resulting `isAtBottomRef` from actual distance. Otherwise, sets `container.scrollTop = container.scrollHeight`, captures the post-clamp value into `lastProgrammaticBottomScrollRef`, and sets `isAtBottomRef.current = true`. On subsequent message arrivals (`messages.length > prev`), if `isAtBottomRef.current`, smooth-scrolls via `bottomRef.scrollIntoView({ behavior: 'smooth' })` — this path deliberately does *not* update the sentinel, because the smooth animation lands asynchronously across many frames and no single intermediate `scrollTop` is worth pinning to. The path is already gated on `isAtBottomRef.current`, so it cannot fire while the user is scrolled away; the rare image-load growth during a smooth scroll is tolerated.
 
 **Effect B — ResizeObserver.** Observes the message-list content container. When height grows and `isAtBottomRef.current === true`, re-pins to bottom and updates the sentinel. Gated on `isAtBottomRef.current` so it cannot interfere when the user has scrolled away.
 

packages/web/src/components/chat/MessageList.tsx

@@ -240,7 +240,7 @@ export function MessageList({ channelId, jumpToMessageId, onJumpComplete }: Mess
     // was queued by our own command. Layout may have grown between the command and the
     // event firing, but our intent is "stay at bottom" — do not let a post-growth distance
     // measurement flip the at-bottom flags. Re-pin defensively (content may have grown
-    // again) and update the sentinel; convergence is described in the spec §2.
+    // again) and update the sentinel. See docs/systems/message-list.md (Auto-scroll model).
     if (container.scrollTop === lastProgrammaticBottomScrollRef.current) {
       isAtBottomRef.current = true;
       setIsAtBottom(true);