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.

Services/ScreenCaptureFrameSource.cs

@@ -64,10 +64,14 @@ public sealed class ScreenCaptureFrameSource : IScreenCaptureSource
     // accommodate in-flight frames) and publishes only monotonically newer frames
     // (MonotonicGate — a slow older completion must never overwrite a newer LatestFrame).
     private static readonly TimeSpan MinConvertInterval = TimeSpan.FromMilliseconds(10);
+    /// <summary>TASK 33: the "shed our own load first" cadence — 30 conversions/s instead of ~100.
+    /// The viewer's tier does not change; only how often we pay for a readback.</summary>
+    private static readonly TimeSpan ShedConvertInterval = TimeSpan.FromMilliseconds(33);
     private static readonly TimeSpan TelemetryInterval = TimeSpan.FromSeconds(2);
     private const int MaxConcurrentConversions = 3;
     private const int PoolBufferCount = 5;
     private DateTime _lastConvertAt = DateTime.MinValue;
+    private TimeSpan _convertInterval = MinConvertInterval;
     private DateTime _telemetryFrom = DateTime.UtcNow;
     private DateTime _lastTelemetry = DateTime.UtcNow;
     private long _skippedCadence;
@@ -79,6 +83,10 @@ public sealed class ScreenCaptureFrameSource : IScreenCaptureSource
     public string Key { get; }
     public event Action<VideoFrame>? FrameAvailable;
 
+    /// <summary>TASK 33: halve our own capture cost before asking the creator to accept a lower tier.
+    /// Idempotent; the pump keeps its declared fps either way (it pads), so no output contract changes.</summary>
+    public void ShedLoad(bool shed) => _convertInterval = shed ? ShedConvertInterval : MinConvertInterval;
+
     internal ScreenCaptureFrameSource(string key, GraphicsCaptureItem item)
     {
         Key = key;
@@ -171,7 +179,7 @@ public sealed class ScreenCaptureFrameSource : IScreenCaptureSource
                 return;
             }
             var now = DateTime.UtcNow;
-            if (now - _lastConvertAt < MinConvertInterval)
+            if (now - _lastConvertAt < _convertInterval)
             {
                 _skippedCadence++;
                 frame.Dispose();