feat(stream): probe the ingest before going live — no more ghost broadcasts (TASK 32·5)

"I forgot the VPN" used to create a broadcast on YouTube that nothing ever pushed to: the app looked
live, YouTube held an empty broadcast, and the creator had to work out what happened. Prevention beats
recovery here, so go-live now asks one question first.

- `Services/IngestProbe.cs` — `IIngestProbe` + `TcpIngestProbe`: parse the RTMP URL, connect to the host
  (rtmp → 1935, rtmps → 443) with a 2 s timeout. Any failure answers "not reachable"; the point is a
  fast unambiguous answer, not a diagnosis.
- `MainViewModel.IngestPreflightAsync` runs **before the first API call** whenever a cached ingest URL
  exists; on a first run it probes once the stream is resolved, still before any broadcast is created.
  A false answer logs, tells the creator ("Nothing was created on YouTube…"), and rolls the session
  back — nothing to clean up on YouTube.
- The probe is a test seam (`IngestProbeOverride`) like the camera and chroma ones.

Tests: `IngestPreflightTests` — unreachable aborts, reachable continues, URL parsing (rtmp/rtmps,
garbage), and the probe against **real sockets**: a listening port answers true, the same port closed
answers false and does so fast. FAST gate: 547 passed, 0 failed, 0 warnings.

ViewModels/MainViewModel.cs

@@ -505,6 +505,9 @@ public partial class MainViewModel : ViewModelBase
     /// a clip's backdrop.</summary>
     internal static IChromaProbe? ChromaProbeOverride { get; set; }
 
+    /// <summary>Test seam (TASK 32·5): a fake ingest probe so tests never open a socket.</summary>
+    internal static IIngestProbe? IngestProbeOverride { get; set; }
+
     private void LoadLayout()
     {
         _isLoading = true;