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.

ytLive.Tests/FramePumpTests.cs

@@ -298,18 +298,23 @@ public class FramePumpTests
     }
 
     [Fact]
-    public async Task ProcessDeath_StopsPump_AndRaisesFailed()
+    public async Task ProcessDeath_StopsPump_AndRaisesEncoderFailed()
     {
         var encoder = new FakeEncoder();
-        var failed = new TaskCompletionSource<string>(TaskCreationOptions.RunContinuationsAsynchronously);
+        var encoderFailed = new TaskCompletionSource<string>(TaskCreationOptions.RunContinuationsAsynchronously);
+        var fatal = false;
         using var pump = NewPump(encoder);
-        pump.Failed += (_, m) => failed.TrySetResult(m);
+        // TASK 32·1: a dead push is recoverable — the broadcast may still be live — so it is a
+        // separate signal from a fatal pump fault, and the session gets to relaunch it.
+        pump.EncoderFailed += (_, m) => encoderFailed.TrySetResult(m);
+        pump.Failed += (_, _) => fatal = true;
 
         await pump.StartAsync();
         encoder.RaiseProcessFailed("FFmpeg exited with code 1");
 
-        var message = await failed.Task.WaitAsync(TimeSpan.FromSeconds(5));
+        var message = await encoderFailed.Task.WaitAsync(TimeSpan.FromSeconds(5));
         Assert.Contains("1", message);
+        Assert.False(fatal, "an encoder death must not be reported as a fatal pump failure");
 
         // The pump self-stops (fire-and-forget); poll the definitive teardown
         // marker — the encoder's disposal — rather than the IsRunning flag, which