docs(ai-chat): document load-bearing invariant of messageSignature memo

PR #182 review (post-fix pass) surfaced two latent correctness risks in the
new MessageItem memo: the per-message signature tracks only [type, text length,
state, error/output presence] + metadata, so a part kind whose VISIBLE content
can change WITHOUT changing those fields would silently freeze a stale row.
Neither is reachable with the current toolset (tool output is set once;
streaming is append-only with a fixed id), so the correct fix is to harden the
documented invariant rather than hash output content on every delta (getPage
returns full page content — hashing it per-delta would tax the hot path this
PR optimizes).

Add a WARNING in messageSignature naming the two future triggers (a tool that
streams `preliminary` output; a client-side regenerate/edit that mutates a
finalized row in place) and the required action (extend the signature).

No behavior change (comment only). vitest src/features/ai-chat 189/189 pass,
tsc clean for the touched files.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/client/src/features/ai-chat/utils/message-signature.ts

@@ -5,7 +5,11 @@ import type { UIMessage } from "@ai-sdk/react";
  *  appended, a tool/text part flips state once), so a per-part [type, text
  *  length, state, error/output presence] tuple + the persisted metadata
  *  (error/finishReason) is a sufficient change signal without comparing full
- *  strings on every delta. */
+ *  strings on every delta. WARNING — load-bearing for the MessageItem memo:
+ *  if a future part kind's VISIBLE content can change WITHOUT changing [type,
+ *  text length, state, error/output presence] (e.g. a tool that streams
+ *  `preliminary` output, or a client-side regenerate that edits a finalized
+ *  row in place), extend this signature or the memo will freeze a stale row. */
 export function messageSignature(message: UIMessage): string {
   const parts = message.parts
     .map((p) => {