feat(33): shed our own capture cost first; the task is done

Last slice. 590 green, 0 warnings.

- **Shed before you charge the viewer.** On machine evidence the policy's first act is
  `ShedCaptureCadence`: `ScreenCaptureFrameSource` widens its conversion interval 10 ms → 33 ms (≈100
  → 30 conversions/s) through a new `ScreenCaptureManager.ShedLoad`. The output tier, the declared fps
  and the viewer's picture are untouched — we stop paying for readbacks we were throwing away. It
  costs no step budget; if the starvation persists, the fps ladder runs on the next clean window.
- The shed is reset on every stop path, so a cheap session cannot leave a later one capped.
- **The session line is a Logs-tab line, not a session-report line.** The session report is the money
  summary (Super Chats, per currency); quality chatter there would bury the numbers it exists for. The
  cause-tagged line already lands in the log at the moment of the step.

TASK 33 is complete: two ladders, budgets, floors, ambiguity, one-way with recovery behind a flag, the
relaunch, the copy, and the cost shed. 18 tests pin the ladders, the copy and the restart.

ViewModels/MainViewModel.StepDown.cs

@@ -39,6 +39,7 @@ public partial class MainViewModel
         _stepDownPlanner = null;
         _encoderTierOverride = null;
         _stepDownBusy = false;
+        _screenCaptureManager.ShedLoad(false);   // never carry a shed cadence into the next show
     }
 
     /// <summary>Runs on the PUMP thread, so it must never block: the restart it may trigger stops the
@@ -64,6 +65,13 @@ public partial class MainViewModel
         var landed = planner.Current;
         try
         {
+            if (decision.Action == StepAction.ShedCaptureCadence)
+            {
+                _screenCaptureManager.ShedLoad(true);
+                AppLog.Write($"Capture load shed: {decision.Why}");
+                return;
+            }
+
             _encoderTierOverride = landed;
             AppLog.Write($"Quality step-down: {decision.Why}");
             await _framePump.ChangeEncoderAsync();