feat(33): the step-down acts — one encoder relaunch, one toast, same URL

Slice 3. 589 green, 0 warnings.

- **`FramePump.ChangeEncoderAsync`** is a full pump stop/start, not an encoder swap. A swap leaves an
  in-flight submit pointing at a disposed process, and that faults the loop — which ends the
  broadcast, the one thing a quality step-down must never do. `StopAsync` already awaits the loop and
  flushes, so this reuses the tested path and costs one short freeze, which the ladder accepts.
- **`MainViewModel.StepDown`**: one planner per broadcast, subscribed to `StatsReported`, unsubscribed on
  every stop path. The pump thread only decides; the restart is posted to the dispatcher, because the
  act stops the loop that raised the event.
- **The creator's choice is not rewritten**: `_encoderTierOverride` changes what the encoder emits while
  the dropdown keeps showing what they picked, and the badge adds `· lowered` so the two never disagree
  silently.
- Copy comes from `StepDownPlanner.Describe` (machine cause owns it, upload cause blames the upload);
  the log gets the cause-tagged line for support.

Test: `EncoderRestartTests` runs a real pump with a fake encoder, changes the tier, and asserts a NEW
encoder started at the new fps/bitrate and produced a frame — the regression that would otherwise ship
as "it lowered the badge and the stream died".

Remaining for 33: shed our own capture cost before the fps step, and the session-report line.

ViewModels/MainViewModel.StepDown.cs

@@ -0,0 +1,94 @@
+using System;
+using System.Windows;
+using ytLive.Helpers;
+using ytLive.Models;
+using ytLive.Services;
+using ytLive.Services.Encoder;
+
+namespace ytLive.ViewModels;
+
+/// <summary>
+/// TASK 33, slice 3: the session's step-down watch — policy feed, one encoder relaunch, one toast.
+///
+/// <para>Kept in its own file because it is the only place that both reads the pump's stats and acts on
+/// the session; the pure decision lives in <see cref="StepDownPolicy"/>, the sentence in
+/// <see cref="StepDownPlanner"/>.</para>
+/// </summary>
+public partial class MainViewModel
+{
+    private StepDownPlanner? _stepDownPlanner;
+    private (int Width, int Height, int Fps, double BitrateMbps)? _encoderTierOverride;
+    private bool _stepDownBusy;
+
+    /// <summary>Starts the watch for this broadcast. Called once per session, before the pump starts.</summary>
+    private void StartStepDownWatch()
+    {
+        var quality = OutputProfile.Resolve(SelectedQuality, QualityOptions, HasProOutput);
+        _encoderTierOverride = null;
+        _stepDownPlanner = new StepDownPlanner(
+            new StepDownPolicy(quality.Width, quality.Height, quality.Fps, quality.Bitrate),
+            _sessionStartedAtUtc ?? DateTime.UtcNow);
+        _framePump.StatsReported += OnPumpStatsReported;
+    }
+
+    /// <summary>Ends the watch. Called from StopStream on every path, so a stale planner can never
+    /// carry a spent budget into the next broadcast.</summary>
+    private void StopStepDownWatch()
+    {
+        _framePump.StatsReported -= OnPumpStatsReported;
+        _stepDownPlanner = null;
+        _encoderTierOverride = null;
+        _stepDownBusy = false;
+    }
+
+    /// <summary>Runs on the PUMP thread, so it must never block: the restart it may trigger stops the
+    /// very loop that raised this event. Decide here (pure and fast), act on the dispatcher.</summary>
+    private void OnPumpStatsReported(object? sender, FramePumpStats stats)
+    {
+        if (_stepDownPlanner is not { } planner) return;
+
+        var decision = planner.Observe(stats, DateTime.UtcNow);
+        if (!decision.Acted) return;
+
+        var dispatcher = Application.Current?.Dispatcher;
+        if (dispatcher == null) return;
+        _ = dispatcher.InvokeAsync(() => ApplyStepDownAsync(decision));
+    }
+
+    private async void ApplyStepDownAsync(StepDownDecision decision)
+    {
+        var planner = _stepDownPlanner;
+        if (planner == null || _stepDownBusy) return;
+
+        _stepDownBusy = true;
+        var landed = planner.Current;
+        try
+        {
+            _encoderTierOverride = landed;
+            AppLog.Write($"Quality step-down: {decision.Why}");
+            await _framePump.ChangeEncoderAsync();
+
+            // Show what is actually going out. The dropdown keeps the creator's choice — the policy
+            // does not get to rewrite their intent, only the encoder's output.
+            CurrentHealth.FPS = landed.Fps;
+            CurrentHealth.CurrentBitrate = landed.BitrateMbps;
+            ResolutionBadgeText = $"{landed.Width}×{landed.Height} @ {landed.Fps} fps · lowered";
+            OnPropertyChanged(nameof(CurrentHealth));
+            OnPropertyChanged(nameof(ResolutionBadgeText));
+
+            var copy = StepDownPlanner.Describe(decision, landed);
+            if (copy.Length > 0)
+                _notifications.Info("Quality lowered", copy);
+
+            AppLog.Write($"Quality step-down complete: {StepDownPlanner.ReportLine(DateTime.Now, decision)}");
+        }
+        catch (Exception ex)
+        {
+            AppLog.Write($"Quality step-down failed: {ex.Message}", AppLogMessageType.Error, AppLogErrorClass.Class2);
+        }
+        finally
+        {
+            _stepDownBusy = false;
+        }
+    }
+}

ViewModels/MainViewModel.Streaming.Operations.cs

@@ -163,6 +163,7 @@ public partial class MainViewModel : ViewModelBase
         if (!TryStartAudioPipe()) return;
         _liveElapsed = TimeSpan.Zero;
         _recordLength = TimeSpan.Zero;
+        StartStepDownWatch();
 
         var recordStarted = false;
         if (alsoRecord)
@@ -405,6 +406,7 @@ public partial class MainViewModel : ViewModelBase
     private async void StopStream()
     {
         CancelStreamRecovery();   // a stop wins over a relaunch in flight
+        StopStepDownWatch();
         RemoveAttentionAlert("stream-recovery");
         var wasLive = IsLive;
         var wasRecording = IsRecording;
@@ -494,6 +496,11 @@ public partial class MainViewModel : ViewModelBase
     {
         var url = _rtmpUrlProvider();
         var quality = OutputProfile.Resolve(SelectedQuality, QualityOptions, HasProOutput);
+        // TASK 33: while a session is live, a step-down owns the emitted tier even though the dropdown
+        // still shows what the creator picked. Their intent is not edited by the policy.
+        if (_encoderTierOverride is { } landed)
+            quality = new QualityOption($"{landed.Width}p{landed.Fps}", landed.Fps,
+                landed.BitrateMbps, landed.Width, landed.Height);
         var compositor = BuildCompositorOptions();
         return new EncoderOptions
         {