fix(mobile perf): avoid eager emoji palette scans for prose (#8416)

Ordinary mobile messages eagerly built a set of every community emoji
shortcode and segmented their entire body just to decide whether to use
enlarged emoji rendering. Stop at the first non-emoji grapheme and build
the shortcode set only when a complete shortcode is encountered.
Emoji-only sizing and shortcode matching stay unchanged.

### Validation

- Full `just ci` passed.
- 125 focused detector and message-rendering tests pass; targeted
analysis is clean. The palette-access regression fails on the original
implementation (10,320 unnecessary reads) and passes with the fix. It
also checks that consecutive shortcodes share one palette scan.
- **Full Buzz app, real community data, debug/JIT iOS simulator, not a
rendering fixture:** two paired actual-input captures measured 125–127
µs per original detector call versus under 1 µs with the fix. All 5,200
executions per version agreed across 260 observed prose calls and a
2,580-entry palette. Instrumentation repeated each detector call, so
these captures are excluded from frame comparisons.
- Separate clean-code scrolling runs do **not** establish a reliable
smoothness improvement: quiet original UI-callback p95 was 10.1–12.6 ms
versus 10.1–12.9 ms fixed. A busy fixed run still reached 90.8 ms.
Normal app services stayed enabled; only scrolling was exercised. Live
background refresh, debug mode, host scheduling, and profiler overhead
limit interpretation. No physical-device/release-AOT or end-to-end
interaction-latency claim.

Signed-off-by: Tom Brow <tomb@block.xyz>

mobile/lib/shared/emoji/emoji_only.dart

@@ -44,35 +44,35 @@ bool isEmojiOnlyMessage(
   final trimmed = content.trim();
   if (trimmed.isEmpty) return false;
 
-  final shortcodes = {
-    for (final emoji in customEmoji) emoji.shortcode.toLowerCase(),
-  };
+  Set<String>? shortcodes;
   var sawEmoji = false;
 
   // Grapheme clusters, so a ZWJ family or a flag pair is one unit. Desktop
   // hand-rolls this scan; Dart's `characters` already segments correctly.
   final iterator = trimmed.characters.iterator;
-  final clusters = <String>[];
   while (iterator.moveNext()) {
-    clusters.add(iterator.current);
-  }
+    final cluster = iterator.current;
 
-  var index = 0;
-  while (index < clusters.length) {
-    final cluster = clusters[index];
-
-    if (cluster.trim().isEmpty) {
-      index += 1;
-      continue;
-    }
+    if (cluster.trim().isEmpty) continue;
 
     if (cluster == ':') {
-      final end = clusters.indexOf(':', index + 1);
-      if (end <= index + 1) return false;
-      final shortcode = clusters.sublist(index + 1, end).join().toLowerCase();
-      if (!shortcodes.contains(shortcode)) return false;
+      final shortcode = StringBuffer();
+      var closed = false;
+      while (iterator.moveNext()) {
+        if (iterator.current == ':') {
+          closed = true;
+          break;
+        }
+        shortcode.write(iterator.current);
+      }
+      if (!closed || shortcode.isEmpty) return false;
+      shortcodes ??= {
+        for (final emoji in customEmoji) emoji.shortcode.toLowerCase(),
+      };
+      if (!shortcodes.contains(shortcode.toString().toLowerCase())) {
+        return false;
+      }
       sawEmoji = true;
-      index = end + 1;
       continue;
     }
 
@@ -80,7 +80,6 @@ bool isEmojiOnlyMessage(
       return false;
     }
     sawEmoji = true;
-    index += 1;
   }
 
   return sawEmoji;

mobile/test/shared/emoji/emoji_only_test.dart

@@ -1,3 +1,5 @@
+import 'dart:collection';
+
 import 'package:buzz/shared/custom_emoji/custom_emoji.dart';
 import 'package:buzz/shared/emoji/emoji_only.dart';
 import 'package:flutter_test/flutter_test.dart';
@@ -55,6 +57,34 @@ void main() {
       expect(emojiOnly('hi :buzz:', customEmoji: _customEmoji), isFalse);
     });
 
+    test('scans consecutive shortcodes without skipping following text', () {
+      expect(emojiOnly(':BUZZ::buzz:', customEmoji: _customEmoji), isTrue);
+      expect(emojiOnly('😀:buzz:😀', customEmoji: _customEmoji), isTrue);
+      expect(
+        emojiOnly(':buzz:ordinary text', customEmoji: _customEmoji),
+        isFalse,
+      );
+      expect(
+        emojiOnly(':buzz::unfinished', customEmoji: _customEmoji),
+        isFalse,
+      );
+      expect(emojiOnly(':buzz:::', customEmoji: _customEmoji), isFalse);
+    });
+
+    test('does not inspect the community palette without a shortcode', () {
+      final palette = _CountingEmojiList();
+      expect(
+        emojiOnly('Ordinary message text. ' * 500, customEmoji: palette),
+        isFalse,
+      );
+      expect(emojiOnly('😀 👋', customEmoji: palette), isTrue);
+      expect(emojiOnly('text :buzz:', customEmoji: palette), isFalse);
+      expect(emojiOnly(':unfinished', customEmoji: palette), isFalse);
+      expect(palette.reads, 0);
+      expect(emojiOnly(':buzz::BUZZ:', customEmoji: palette), isTrue);
+      expect(palette.reads, palette.length);
+    });
+
     test('an unknown shortcode is just text', () {
       // Desktop behaves the same: without the palette it renders literally, so
       // it must not be scaled up as if it were a glyph.
@@ -79,3 +109,26 @@ void main() {
     });
   });
 }
+
+class _CountingEmojiList extends ListBase<CustomEmoji> {
+  int reads = 0;
+
+  @override
+  int get length => 2580;
+
+  @override
+  set length(int value) => throw UnsupportedError('read-only fixture');
+
+  @override
+  CustomEmoji operator [](int index) {
+    reads += 1;
+    return CustomEmoji(
+      shortcode: index == 0 ? 'buzz' : 'unused_$index',
+      url: 'https://example.test/emoji.png',
+    );
+  }
+
+  @override
+  void operator []=(int index, CustomEmoji value) =>
+      throw UnsupportedError('read-only fixture');
+}