feat(stream): retry a dropped push instead of ending the session (TASK 32·1)

A dead encoder used to run the whole stop path, so a one-second blip ended the broadcast. Now a dropped
push is relaunched against the same ingest while the broadcast is still alive.

- `Services/StreamRetryPolicy.cs` — immediate, +1 s, +2 s, +3 s. Minimum waits on purpose (creator
  ruling): YouTube's grace is short, so every second not pushing is grace spent.
- `Services/StreamRecovery.cs` — the loop, with the audio pipe recreated before every attempt (its
  server is single-instance and unusable after the old client dies, so a naive relaunch fails on a
  missing input). After a push comes up it asks the API whether the broadcast survived: "complete"
  ends recovery as `BroadcastComplete` instead of showing a live sign over a dead stream; an API
  hiccup counts as recovered rather than ending a healthy session.
- `FramePump.EncoderFailed` — a dead push is recoverable; a compositor fault still goes through
  `Failed` and ends the session, because retrying it would loop.
- `MainViewModel.Recovery.cs` — wires the steps, reports each episode into the left panel alert stack
  (one row per episode, updated in place, log range painted), and cancels on Stop.
- Alert stack polish: a rule and a gap now separate the stack from the scene layers (they read as one
  list before), and the licence indicator uses the creator's wording — `STEAM ID NOT PRESENT` /
  `Steam ID Limit: 30fps` / `Click for More Info`, becoming `STEAM ID PRESENT` / `60fps` with a close
  control once owned.

Tests: `StreamRecoveryTests` (schedule, recover on attempt 2 with the pipe restarted each time,
complete-broadcast honesty, unknown liveness, give-up, cancel, audio-restart fault),
`FramePumpTests` now pin that process death raises `EncoderFailed` and not `Failed`.
FAST gate: 538 passed (one documented pre-existing audio failure), 0 warnings.

ViewModels/MainViewModel.Streaming.Operations.cs

@@ -339,6 +339,8 @@ public partial class MainViewModel : ViewModelBase
 
     private async void StopStream()
     {
+        CancelStreamRecovery();   // a stop wins over a relaunch in flight
+        RemoveAttentionAlert("stream-recovery");
         var wasLive = IsLive;
         var wasRecording = IsRecording;