feat(33): structured pump stats + the session planner and its plain-words copy

Slice 2. 588 green, 0 warnings.

- `FramePumpStats` and `FramePump.StatsReported`: the 5 s window the log line already describes, as
  data. `Dropped` is the ENCODER's counter (frames submitted, not accepted); `Stalls`/`AvgRenderMs`
  are the pump's. `FramesRendered` is deliberately not used for classification — a static scene
  reuses the paste cache and renders few frames while completely healthy, so it would read as
  starvation.
- `StepDownPlanner`: one per broadcast, owns the stateful policy, converts stats → sample, exposes the
  landed tier for the restart. It never acts — the VM owns the effects, which is what keeps the ladder
  testable without an encoder.
- `Describe` / `ReportLine`: the sentences a creator sees. Machine cause says LlamaCasty couldn't keep
  up and the connection was not the problem; upload cause blames the upload and promises no further
  thrash. A test asserts the machine copy never leaks "dropped" — mechanism strings stay in the log.

Remaining for the task: VM subscription, the encoder restart at the landed tier, the toast, and
shedding our own capture cost before stepping fps.

Services/Encoder/FramePumpStats.cs

@@ -0,0 +1,26 @@
+namespace ytLive.Services.Encoder;
+
+/// <summary>One 5-second window of pump and encoder evidence, structured.
+///
+/// <para>The stats line in the log is for humans; this is for the policy that has to decide whether
+/// the machine or the connection is the problem. Note which counter is which:</para>
+/// <list type="bullet">
+/// <item><see cref="Stalls"/> and <see cref="AvgRenderMs"/> are the MACHINE's evidence — the pump
+/// could not produce a frame in time.</item>
+/// <item><see cref="Dropped"/> is the ENCODER's — frames submitted but not accepted downstream. It
+/// rises when ffmpeg is blocked, which under CBR means the socket, not the renderer.</item>
+/// <item><see cref="FramesRendered"/> is deliberately NOT used for classification: a static scene
+/// legitimately reuses the paste cache and renders few frames while completely healthy.</item>
+/// </list>
+/// </summary>
+public readonly record struct FramePumpStats(
+    double WindowSeconds,
+    int FramesSubmitted,
+    int FramesRendered,
+    double AvgRenderMs,
+    double AvgSubmitMs,
+    double AvgWaitMs,
+    double WorstRenderMs,
+    double WorstSubmitMs,
+    int Dropped,
+    int Stalls);