fix(ui): reconcile the log visualizer tick instead of clearing Rows

The 4s reconcile tick called Apply(), which did Rows.Clear() + refill on
every tick -- resetting the user's scroll position and selection while
they're mid-read, exactly what the overlay exists to avoid.

Reconcile granularly instead: find the overlap between the previous and
latest snapshot (the ring buffer is append-mostly, evicting from the
front as entries age out of the 30-min window), trim rows for evicted
entries off the tail, and insert new entries at the head. A tick with no
change now leaves Rows untouched. The WarnErrorOnly toggle still goes
through the full Apply() rebuild, unchanged.

tests/ClaudeDo.Ui.Tests/ViewModels/LogVisualizerViewModelTests.cs

@@ -13,6 +13,14 @@ public class LogVisualizerViewModelTests
         public override Task<IReadOnlyList<WorkerLogEntry>> GetRecentLogsAsync() => Task.FromResult(_logs);
     }
 
+    // Simulates the worker's append-mostly LogRingBuffer across successive ticks: each
+    // RefreshAsync call returns whatever `Logs` currently holds.
+    private sealed class MutableFakeClient : StubWorkerClient
+    {
+        public IReadOnlyList<WorkerLogEntry> Logs { get; set; } = Array.Empty<WorkerLogEntry>();
+        public override Task<IReadOnlyList<WorkerLogEntry>> GetRecentLogsAsync() => Task.FromResult(Logs);
+    }
+
     private static WorkerLogEntry E(WorkerLogLevel lvl, string msg)
         => new(msg, lvl, new DateTime(2026, 6, 23, 8, 0, 0, DateTimeKind.Utc));
 
@@ -176,4 +184,56 @@ public class LogVisualizerViewModelTests
 
         Assert.False(string.IsNullOrEmpty(vm.StatusText));
     }
+
+    [Fact]
+    public async Task Tick_with_no_new_data_does_not_touch_rows()
+    {
+        var t0 = new DateTime(2026, 6, 23, 8, 0, 0, DateTimeKind.Utc);
+        var client = new MutableFakeClient { Logs = new[] { E(WorkerLogLevel.Info, "a", t0) } };
+        var vm = new LogVisualizerViewModel(client);
+        await vm.RefreshAsync();
+        var row = vm.Rows.Single();
+
+        var changeCount = 0;
+        vm.Rows.CollectionChanged += (_, _) => changeCount++;
+        await vm.RefreshAsync(); // same snapshot on the next tick
+
+        Assert.Equal(0, changeCount);
+        Assert.Same(row, vm.Rows.Single());
+    }
+
+    [Fact]
+    public async Task Tick_appends_a_new_entry_without_clearing_existing_rows()
+    {
+        var t0 = new DateTime(2026, 6, 23, 8, 0, 0, DateTimeKind.Utc);
+        var client = new MutableFakeClient { Logs = new[] { E(WorkerLogLevel.Info, "a", t0) } };
+        var vm = new LogVisualizerViewModel(client);
+        await vm.RefreshAsync();
+        var existingRow = vm.Rows.Single();
+
+        client.Logs = new[] { E(WorkerLogLevel.Info, "a", t0), E(WorkerLogLevel.Info, "b", t0.AddMinutes(1)) };
+        await vm.RefreshAsync();
+
+        Assert.Equal(new[] { "b", "a" }, vm.Rows.Select(r => r.Message));
+        Assert.Same(existingRow, vm.Rows[1]); // the pre-existing row instance survived the tick
+    }
+
+    [Fact]
+    public async Task Tick_drops_the_row_for_an_entry_that_aged_out_of_the_window()
+    {
+        var t0 = new DateTime(2026, 6, 23, 8, 0, 0, DateTimeKind.Utc);
+        var client = new MutableFakeClient
+        {
+            Logs = new[] { E(WorkerLogLevel.Info, "a", t0), E(WorkerLogLevel.Info, "b", t0.AddMinutes(1)) },
+        };
+        var vm = new LogVisualizerViewModel(client);
+        await vm.RefreshAsync();
+        Assert.Equal(new[] { "b", "a" }, vm.Rows.Select(r => r.Message));
+
+        // "a" fell out of the ring buffer's 30-min window; "c" is a new entry.
+        client.Logs = new[] { E(WorkerLogLevel.Info, "b", t0.AddMinutes(1)), E(WorkerLogLevel.Info, "c", t0.AddMinutes(2)) };
+        await vm.RefreshAsync();
+
+        Assert.Equal(new[] { "c", "b" }, vm.Rows.Select(r => r.Message));
+    }
 }