feat(quality): learn the starting rate from real sessions, on the creator's permission

Each finished broadcast is filed locally — tier, probed encoder, rated, settled, steps
spent — and Models/RateHistory turns that into at most a suggestion. The app never
applies one: it asks before the next stream, "Use it" sets the rate, "Keep mine"
remembers that exact number so the same question does not return until the evidence
moves. Nothing leaves the machine.

Two rules keep it from nagging. The high-water mark is the highest rate that
*finished* a session with zero steps, not the highest seen, so one easy session (a
menu screen, a quiet game) cannot set a target the machine cannot hold. And it is
asymmetric: raising needs 10% above the current rate across five clean sessions of the
same tier and encoder, lowering needs three step-downs in five — fixing damage should
not wait for a percentage, and risking it should.

The encoder key is real, not decorative: StreamHealth.VideoEncoder is now filled from
the probe (FfmpegEncoder), so history recorded against NVENC is not applied to a
machine that later runs QSV. At startup, before anything has streamed, the last
session on that tier is the honest guess.

672 pass, 0 warnings.

ViewModels/MainViewModel.StepDown.cs

@@ -19,12 +19,14 @@ public partial class MainViewModel
     private StepDownPlanner? _stepDownPlanner;
     private (int Width, int Height, int Fps, double BitrateMbps)? _encoderTierOverride;
     private bool _stepDownBusy;
+    private double _sessionRatedMbps;
 
     /// <summary>Starts the watch for this broadcast. Called once per session, before the pump starts.</summary>
     private void StartStepDownWatch()
     {
         var quality = EffectiveQuality(OutputProfile.Resolve(SelectedQuality, QualityOptions, HasProOutput));
         _encoderTierOverride = null;
+        _sessionRatedMbps = quality.Bitrate;
         _stepDownPlanner = new StepDownPlanner(
             new StepDownPolicy(quality.Width, quality.Height, quality.Fps, quality.Bitrate,
                 floorBitrateMbps: RateLadder.For(quality).Minimum),
@@ -37,12 +39,35 @@ public partial class MainViewModel
     private void StopStepDownWatch()
     {
         _framePump.StatsReported -= OnPumpStatsReported;
+        RecordSessionRate();
         _stepDownPlanner = null;
         _encoderTierOverride = null;
         _stepDownBusy = false;
         _screenCaptureManager.ShedLoad(false);   // never carry a shed cadence into the next show
     }
 
+    /// <summary>
+    /// One finished session, filed as evidence for the rate-history policy (Models/RateHistory).
+    /// Best effort: a store failure must never be the reason a stop fails.
+    /// </summary>
+    private void RecordSessionRate()
+    {
+        if (_stepDownPlanner is not { } planner) return;
+        try
+        {
+            _layoutStore.AppendRateHistory(new SessionRateResult(
+                SelectedQuality.Display,
+                CurrentHealth.VideoEncoder ?? "unknown",
+                _sessionRatedMbps,
+                planner.Current.BitrateMbps,
+                planner.Policy.StepsUsed));
+        }
+        catch (Exception ex)
+        {
+            AppLog.Write($"Rate history not recorded: {ex.Message}", AppLogMessageType.Warning, AppLogErrorClass.Class2);
+        }
+    }
+
     /// <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)