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.

Models/StepDownPolicy.cs

@@ -18,6 +18,8 @@ public enum StepCause
 public enum StepAction
 {
     None,
+    /// <summary>Cheapest first: halve LlamaCasty's own capture cost, change nothing the viewer sees.</summary>
+    ShedCaptureCadence,
     LowerFrameRate,
     LowerResolution,
     LowerBitrate,
@@ -94,6 +96,7 @@ public sealed class StepDownPolicy
     private double _lastStepAt = double.NegativeInfinity;
     private double _lastRaiseAt = double.NegativeInfinity;
     private bool _frozen;
+    private bool _shedCapture;
 
     public StepDownPolicy(bool vertical, int startFps, double startBitrateMbps, bool allowClimb = false)
         : this(vertical ? 1080 : 1920, vertical ? 1920 : 1080, startFps, startBitrateMbps, allowClimb)
@@ -175,6 +178,17 @@ public sealed class StepDownPolicy
         if (now - _lastStepAt < CooldownSeconds)
             return new StepDownDecision(machine ? StepCause.Machine : StepCause.Upload, StepAction.None, "cooldown");
 
+        // Machine starvation: shed OUR cost before the viewer's tier is touched. Costs no step and is
+        // not a quality change — if it does not fix the starvation, the fps ladder runs next window.
+        if (machine && !_shedCapture)
+        {
+            _shedCapture = true;
+            _qualifyingSeconds = 0;
+            _lastStepAt = now;
+            return new StepDownDecision(StepCause.Machine, StepAction.ShedCaptureCadence,
+                "halving capture cost before lowering quality");
+        }
+
         if (StepsUsed >= StepsAllowed)
             return new StepDownDecision(machine ? StepCause.Machine : StepCause.Upload, StepAction.None,
                 "step budget spent — holding");