fix(edit-history): harden diff after review

Address findings from 2 review agents on the edit-history diff:

- Perf: diffWords is O(n*m); cap at 2000 tokens/side and fall back to a
  coarse whole-block replaced diff above that, so a very large multi-edit
  message can't freeze the main thread. Memoize the per-row diff in
  DiffText. (Added a unit test for the coarse fallback.)

- Perceivability (a11y/design): the added-word <ins> highlight was
  color-fill only, which is faint against the modal surface in the lotus
  themes. Add a Success.ContainerLine border + horizontal padding (so the
  rounded corners read as a chip) + box-decoration-break: clone for clean
  wrapping, so the "added" cue survives low fill contrast.

- Consistency: a media/no-body edit now renders "(no text)" in diff mode
  too (matched the toggle-off view; was blank).

- Softened the code comment's screen-reader claim (bare <ins>/<del> aren't
  announced by default).

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

src/app/features/room/message/EditHistoryModal.tsx

@@ -1,4 +1,4 @@
-import React, { ReactNode, useCallback, useEffect, useState } from 'react';
+import React, { ReactNode, useCallback, useEffect, useMemo, useState } from 'react';
 import parse from 'html-react-parser';
 import Linkify from 'linkify-react';
 import FocusTrap from 'focus-trap-react';
@@ -113,10 +113,11 @@ function getVersionText(evt: MatrixEvent): string {
 }
 
 // Renders a word-level diff of prev -> next: added words highlighted, removed
-// words struck-through. Uses semantic <ins>/<del> so screen readers announce
-// the change. Plain-text only (formatting isn't diffed — see the toggle).
+// words struck-through, using semantic <ins>/<del> elements. Plain-text only
+// (formatting isn't diffed — see the "Highlight changes" toggle).
 function DiffText({ prev, next }: { prev: string; next: string }) {
-  const segments = diffWords(prev, next);
+  const segments = useMemo(() => diffWords(prev, next), [prev, next]);
+  if (segments.length === 0) return <>(no text)</>;
   return (
     <>
       {segments.map((seg, i) => {
@@ -128,8 +129,12 @@ function DiffText({ prev, next }: { prev: string; next: string }) {
               style={{
                 background: color.Success.Container,
                 color: color.Success.OnContainer,
+                border: `${config.borderWidth.B300} solid ${color.Success.ContainerLine}`,
                 borderRadius: config.radii.R300,
+                padding: `0 ${config.space.S100}`,
                 textDecoration: 'none',
+                boxDecorationBreak: 'clone',
+                WebkitBoxDecorationBreak: 'clone',
               }}
             >
               {seg.text}