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.

ytLive.Tests/StepDownPlannerTests.cs

@@ -26,7 +26,7 @@ public sealed class StepDownPlannerTests
             new StepDownPolicy(vertical: false, startFps: 60, startBitrateMbps: 8.0), now);
 
         StepDownDecision decision = default;
-        for (var i = 0; i < 4 && !decision.Acted; i++)
+        for (var i = 0; i < 20 && !(decision.Acted && decision.Action != StepAction.ShedCaptureCadence); i++)
         {
             now = now.AddSeconds(5);
             decision = planner.Observe(Strained(), now);