Slice 3. 589 green, 0 warnings.
- **`FramePump.ChangeEncoderAsync`** is a full pump stop/start, not an encoder swap. A swap leaves an
in-flight submit pointing at a disposed process, and that faults the loop — which ends the
broadcast, the one thing a quality step-down must never do. `StopAsync` already awaits the loop and
flushes, so this reuses the tested path and costs one short freeze, which the ladder accepts.
- **`MainViewModel.StepDown`**: one planner per broadcast, subscribed to `StatsReported`, unsubscribed on
every stop path. The pump thread only decides; the restart is posted to the dispatcher, because the
act stops the loop that raised the event.
- **The creator's choice is not rewritten**: `_encoderTierOverride` changes what the encoder emits while
the dropdown keeps showing what they picked, and the badge adds `· lowered` so the two never disagree
silently.
- Copy comes from `StepDownPlanner.Describe` (machine cause owns it, upload cause blames the upload);
the log gets the cause-tagged line for support.
Test: `EncoderRestartTests` runs a real pump with a fake encoder, changes the tier, and asserts a NEW
encoder started at the new fps/bitrate and produced a frame — the regression that would otherwise ship
as "it lowered the badge and the stream died".
Remaining for 33: shed our own capture cost before the fps step, and the session-report line.
