fix(mobile): keep unrestored escape stand-ins readable

gpt_markdown sometimes shows text unparsed, for example a strikethrough
that spans lines, and then a private-use stand-in showed as nothing.
Make each stand-in a WORD JOINER followed by the character's fullwidth
form, so that fallback still reads `＃` rather than losing the character.

Co-authored-by: studio <dc2fc481343672df31a689840c09299972e704dac592e0da6f18486646a7ee3e@buzz.block.builderlab.xyz>
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tom Brow <tomb@squareup.com>

mobile/lib/features/channels/message_content/markdown_escapes.dart

@@ -1,18 +1,19 @@
 import 'link_normalizer.dart';
 
-/// Holds each backslash-escaped character as a private-use stand-in, so no
-/// link, emphasis, mention, emoji, channel or math syntax matches it: `\*x\*`
-/// stays `*x*` and `\@alice` stays text.
+/// Holds each backslash-escaped character as a stand-in, so no link,
+/// emphasis, mention, emoji, channel or math syntax matches it: `\*x\*` stays
+/// `*x*` and `\@alice` stays text.
 ///
-/// A stand-in is [_marker] followed by the character shifted into the
-/// private-use area. An authored [_marker] becomes two markers, so authored
-/// private-use characters come back from [unshieldMarkdownEscapes] unchanged.
+/// A stand-in is an invisible [_marker] followed by the character's fullwidth
+/// form, so text the renderer shows without restoring it still reads right.
+/// An authored [_marker] becomes two markers, so authored text comes back from
+/// [unshieldMarkdownEscapes] unchanged.
 String shieldMarkdownEscapes(String text) => _needsShield(text)
     ? text.replaceAllMapped(
         _escape,
         (m) => m[1] == null
             ? '$_marker$_marker'
-            : '$_marker${String.fromCharCode(_escapeBase + m[1]!.codeUnitAt(0))}',
+            : '$_marker${String.fromCharCode(_fullwidthOffset + m[1]!.codeUnitAt(0))}',
       )
     : text;
 
@@ -34,18 +35,20 @@ String reescapeMarkdownEscapes(String text) => text.replaceAllMapped(
 );
 
 /// Matches one stand-in created by [shieldMarkdownEscapes].
-final shieldedMarkdownEscape = RegExp('\uE000[\uE000\uE021-\uE07E]');
+final shieldedMarkdownEscape = RegExp('\u2060[\u2060\uFF01-\uFF5E]');
 
 bool _needsShield(String text) => text.contains(r'\') || text.contains(_marker);
 
 String _held(String standIn) {
-  final unit = standIn.codeUnitAt(1);
-  return unit == _escapeBase
+  final held = standIn.substring(1);
+  return held == _marker
       ? _marker
-      : String.fromCharCode(unit - _escapeBase);
+      : String.fromCharCode(held.codeUnitAt(0) - _fullwidthOffset);
 }
 
-const _marker = '\uE000';
-const _escapeBase = 0xE000;
+// WORD JOINER: zero-width, not whitespace, and not Markdown syntax.
+const _marker = '\u2060';
+// ASCII 0x21-0x7E to FULLWIDTH 0xFF01-0xFF5E.
+const _fullwidthOffset = 0xFEE0;
 // CommonMark: a backslash escapes any ASCII punctuation character.
-final _escape = RegExp(r'\\([!-/:-@\[-`{-~])|\uE000');
+final _escape = RegExp(r'\\([!-/:-@\[-`{-~])|\u2060');

mobile/test/features/channels/message_content_test.dart

@@ -512,8 +512,10 @@ void main() {
         ('```\nx \\# y\n```', r'x \# y'),
         (r'code `a\#b`', r'a\#b'),
         (r'\`literal\` then `a\#b`', '`literal` then '),
-        // Authored private-use characters are not mistaken for escapes.
-        ('\uE02A \uE000\uE02A \\#', '\uE02A \uE000\uE02A #'),
+        // Authored stand-in characters are not mistaken for escapes.
+        ('\uFF0A \u2060\uFF0A \\#', '\uFF0A \u2060\uFF0A #'),
+        // gpt_markdown shows this unparsed; the stand-in still reads as #.
+        ('~~a\nb \\#c~~', '\uFF03c'),
         // A bare link's trailing backslash stays outside the link.
         (r'see https://example.com/a\', 'see '),
         (r'in buzz://channel/9f18f3c6-b5e5-453d-b36d-65487913be0b.\', r'.\'),
@@ -538,17 +540,17 @@ void main() {
             );
           }
           // No stand-in or leaked link syntax reaches text or semantics.
-          if (content.contains('\uE000')) continue;
+          if (content.contains('\u2060') || shown == '\uFF03c') continue;
           expect(
             find.textContaining(
-              RegExp('\uE000|\\]\\(http'),
+              RegExp('\u2060|\\]\\(http'),
               findRichText: true,
             ),
             findsNothing,
             reason: reason,
           );
           expect(
-            find.bySemanticsLabel(RegExp('\uE000')),
+            find.bySemanticsLabel(RegExp('\u2060')),
             findsNothing,
             reason: reason,
           );