perf(mobile): parse each event's thread reference once (#9460)

Every time code asked an event for its parent or thread root
(`threadReference`, `parentEventId`), `parseThreadReference` walked its
tags again and ran a 64-character hex regex over each `e` tag. The
channel store asks that many times per event:

- `_trimReplyOverlay` runs on every live event merged into the window
and re-parsed every overlay event, twice.
- `_reconcilePageSummaries` called `cachedThreadReplyIds(root)` once per
row of a fetched page, and each call re-parsed every cached reply (twice
per reply) to find that root's replies. A 200-row page did 200 full
passes.

Now:

- `parseThreadReference` keeps its result per tag list in an `Expando`.
Event tags are never changed after an event is built (nothing in `lib`
mutates a `NostrEvent`'s tags), so the cached value always matches.
- `_reconcilePageSummaries` indexes cached replies by root once, then
reads each row's count from that map. `cachedThreadReplyIds` keeps its
signature and result and shares the same code.

### Validation

Full mobile test suite and `flutter analyze` pass. Existing
thread-summary, reply-overlay and thread-reference tests cover the
unchanged results.

On the iPhone 12 mini (profile build, Builderlab), same scripted run on
`main` and on this branch: open three threads from Activity, back out
after each.

| Dart CPU in thread-reference code | main | this branch |
| --- | --- | --- |
| First thread (not cached yet) | 5 ms | 2 ms |
| Second thread (loads the channel window) | 27 ms, plus 18 ms trimming
the reply overlay | 3 ms, plus 2 ms |
| Third thread | 10 ms | 0 ms |

The threads opened and showed the same replies in both runs, and the run
covered a channel window load, live events arriving while a thread was
open, and backing out to Home.

### Regressions

None expected. Results are identical; the cache holds one small record
per live tag list and is released with the event.

Signed-off-by: Tom Brow <tomb@squareup.com>
Co-authored-by: Tom Brow <tomb@squareup.com>
Co-authored-by: studio <dc2fc481343672df31a689840c09299972e704dac592e0da6f18486646a7ee3e@buzz.block.builderlab.xyz>

mobile/lib/features/channels/channel_messages_provider.dart

@@ -624,7 +624,11 @@ class ChannelMessagesNotifier extends Notifier<AsyncValue<List<NostrEvent>>> {
       _rotateDeletionProofTargets(missingIds);
 
   /// Snapshots cached overlay replies covered by a query for this outer root.
-  Set<String> cachedThreadReplyIds(String rootId) {
+  Set<String> cachedThreadReplyIds(String rootId) =>
+      _cachedThreadReplyIdsByRoot(only: rootId)[rootId] ?? <String>{};
+
+  /// Snapshots cached overlay replies for every outer root, or only [only].
+  Map<String, Set<String>> _cachedThreadReplyIdsByRoot({String? only}) {
     final deletedIds = {
       for (final event in [
         ..._windowStore.liveAux,
@@ -636,15 +640,21 @@ class ChannelMessagesNotifier extends Notifier<AsyncValue<List<NostrEvent>>> {
           for (final tag in event.tags)
             if (tag.length > 1 && tag[0] == 'e') tag[1],
     };
-    return {
-      for (final entry in _localReplyRoots.entries)
-        if (entry.value == rootId && !deletedIds.contains(entry.key)) entry.key,
-      for (final event in _cachedThreadEvents)
-        if (event.threadReference.parentId != null &&
-            event.threadReference.rootId == rootId &&
-            !deletedIds.contains(event.id))
-          event.id,
-    };
+    final byRoot = <String, Set<String>>{};
+    void add(String root, String id) {
+      if ((only == null || root == only) && !deletedIds.contains(id)) {
+        (byRoot[root] ??= <String>{}).add(id);
+      }
+    }
+
+    for (final entry in _localReplyRoots.entries) {
+      add(entry.value, entry.key);
+    }
+    for (final event in _cachedThreadEvents) {
+      final thread = event.threadReference;
+      if (thread.parentId != null) add(thread.rootId!, event.id);
+    }
+    return byRoot;
   }
 
   /// Applies explicit deletion evidence fetched for cached replies.

mobile/lib/features/channels/channel_messages_provider/thread_summaries.dart

@@ -2,6 +2,8 @@ part of '../channel_messages_provider.dart';
 
 extension _ThreadSummaryState on ChannelMessagesNotifier {
   void _reconcilePageSummaries(ChannelWindowPage page, int pageVersion) {
+    // Nothing below changes the cached replies, so index them once per page.
+    final cachedReplies = _cachedThreadReplyIdsByRoot();
     for (final row in page.rows) {
       final root = row.event.id;
       if ((_threadEvidenceVersions[root] ?? 0) > pageVersion) continue;
@@ -10,7 +12,7 @@ extension _ThreadSummaryState on ChannelMessagesNotifier {
       _summaryRefreshes.cancel(root);
       _setThreadQueryVersion(root, pageVersion);
       _clearDeletionUncertainty(root, pageVersion);
-      if (cachedThreadReplyIds(root).length >
+      if ((cachedReplies[root]?.length ?? 0) >
           (row.thread?.descendantCount ?? 0)) {
         _queueOverflowSummary(root);
       }

mobile/lib/shared/relay/thread_reference.dart

@@ -3,8 +3,18 @@
 /// The last valid 64-character ASCII-hex marker wins. IDs are lowercased for
 /// client lookup. Root-only events remain top-level; a reply without a valid
 /// root uses its parent as root.
+///
+/// Timeline and thread-summary code reads this many times per event, so the
+/// result is kept per tag list. Event tags are never changed after an event is
+/// built.
 ({String? parentId, String? rootId}) parseThreadReference(
   List<List<String>> tags,
+) => _threadReferences[tags] ??= _parseThreadReference(tags);
+
+final _threadReferences = Expando<({String? parentId, String? rootId})>();
+
+({String? parentId, String? rootId}) _parseThreadReference(
+  List<List<String>> tags,
 ) {
   String? parentId;
   String? rootId;