perf(mobile): evict verified-profile interest in one pass (#9007)

## Summary

Follow-up to [#8931](https://github.com/block/buzz/pull/8931), from
Wes's [review
note](https://github.com/block/buzz/pull/8931#discussion_r4456891410).

[#8931](https://github.com/block/buzz/pull/8931) replaced the
per-eviction scan in `VerifiedProfilesNotifier.track` with `keys.first`.
That was much cheaper than main, but not constant time: removing from a
Dart map leaves deleted slots, and each new `keys` iterator walks past
them. One `track` call that overflows the 1,024-subject cap by E
subjects still costs about E²/2 slot checks. Wes measured ~3 ms at 4,300
subjects, 62 ms at 8,600 and 205 ms at 17,200.

Eviction now takes the overflow from a single iteration
(`keys.take(overflow).toList()`), so it's linear. Which subjects are
kept doesn't change: still the 1,024 most recently inserted.

The [#8931](https://github.com/block/buzz/pull/8931) eviction test
counted keys yielded by the map, which `keys.first` keeps at one per
eviction, so it couldn't fail on the quadratic path. It now counts
iterations started over the interest map and expects exactly one per
`track` call. Main's scan and the merged `keys.first` loop both start
one per eviction.

### Testing
- Falsification: with the merged `keys.first` loop restored, the new
test fails (`Expected: <1>, Actual: <3276>`). With this change it
passes.
- `flutter test test/shared/verified_profiles/`: 31 passed. The existing
"keeps the 1,024 most recently tracked subjects" journey still passes,
so retention is unchanged.
- `just mobile-check`: clean.
- `just mobile-test`: 4,227 Flutter tests passed, 40 skipped, plus 4
push and 27 crypto tests.
- Automated app run: installed this build on an iOS 27 simulator (iPhone
17 Pro) signed in to a Builderlab community, opened the inbox, a channel
timeline with images and the image viewer, and the channel details
member list, which all track verified-profile subjects. No errors. That
community is small (30 members), so the overflow path itself is covered
by the unit tests, not the app run.

- Agent review: an independent agent review found no blockers.
- Human test: waived under the mobile testing exception; the automated
app run above replaces it.

Not covered: Android, and no new device profile (Wes's AOT numbers above
are the cost model; this removes the E² term).

buzz-review-completed

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

mobile/lib/shared/verified_profiles/verified_profile_directory.dart

@@ -134,13 +134,17 @@ class VerifiedProfilesNotifier extends Notifier<Map<String, VerifiedProfile>> {
       _wanted[key] ??= ++_interest;
     }
     // A key's interest is set once, when it is inserted, so the map's
-    // insertion order is interest order and the oldest key is the first. A
-    // scan per eviction made one 4,000-member roster take ~300 ms on device.
-    while (_wanted.length > _wantedLimit) {
-      final oldest = _wanted.keys.first;
-      _wanted.remove(oldest);
-      _heads.remove(oldest);
-      _checked.remove(oldest);
+    // insertion order is interest order and the oldest keys come first. Take
+    // them in one pass: a scan per eviction made one 4,000-member roster take
+    // ~300 ms on device, and even `keys.first` per eviction is quadratic,
+    // because each new iterator skips the slots earlier removals left behind.
+    final overflow = _wanted.length - _wantedLimit;
+    if (overflow > 0) {
+      for (final oldest in _wanted.keys.take(overflow).toList()) {
+        _wanted.remove(oldest);
+        _heads.remove(oldest);
+        _checked.remove(oldest);
+      }
       added = true;
     }
     if (!added) return;

mobile/test/shared/verified_profiles/verified_profile_directory_test.dart

@@ -14,11 +14,13 @@ import 'package:hooks_riverpod/hooks_riverpod.dart';
 import '../../helpers/fake_relay.dart';
 import 'verified_profile_fixture.dart';
 
-/// Counts the keys read through [keys], which [entries] and [values] also use.
+/// Counts the iterations started over [keys], which [entries] and [values]
+/// also use. Each one costs a walk past every slot that earlier removals left
+/// behind, so one per eviction is quadratic even when it stops at the first key.
 class _CountingMap extends MapBase<String, int> {
-  _CountingMap(this.visits);
+  _CountingMap(this.iterations);
 
-  final void Function() visits;
+  final void Function() iterations;
   final Map<String, int> _inner = {};
 
   @override
@@ -39,20 +41,18 @@ class _CountingMap extends MapBase<String, int> {
   bool containsKey(Object? key) => _inner.containsKey(key);
   @override
   Iterable<String> get keys sync* {
-    for (final key in _inner.keys) {
-      visits();
-      yield key;
-    }
+    iterations();
+    yield* _inner.keys;
   }
 }
 
 class _CountingNotifier extends VerifiedProfilesNotifier {
-  _CountingNotifier(this.visits);
+  _CountingNotifier(this.iterations);
 
-  final void Function() visits;
+  final void Function() iterations;
 
   @override
-  Map<String, int> newInterestMap() => _CountingMap(visits);
+  Map<String, int> newInterestMap() => _CountingMap(iterations);
 }
 
 class Harness {
@@ -109,23 +109,23 @@ void journey(String name, void Function(Harness h) body) {
 }
 
 void main() {
-  test('evicting past the interest limit does not scan the interest map', () {
+  test('evicting past the interest limit iterates the interest map once', () {
     fakeAsync((async) {
-      var visits = 0;
+      var iterations = 0;
       final h = Harness(
         async,
-        notifier: () => _CountingNotifier(() => visits++),
+        notifier: () => _CountingNotifier(() => iterations++),
       );
       addTearDown(h.container.dispose);
       String key(int i) => i.toRadixString(16).padLeft(64, '0');
       h.start([for (var i = 0; i < 1024; i++) key(i)]);
-      visits = 0;
-      // Each new subject evicts one: 3,276 evictions. Scanning the map for the
-      // oldest entry each time would visit about a thousand keys apiece.
+      iterations = 0;
+      // 3,276 evictions. Starting an iteration per eviction, whether to scan
+      // for the oldest entry or to read the first key, is quadratic.
       h.container.read(verifiedProfilesProvider.notifier).track([
         for (var i = 1024; i < 4300; i++) key(i),
       ]);
-      expect(visits, lessThanOrEqualTo(4300 - 1024));
+      expect(iterations, 1);
     });
   });