debug(audience): add T1/T2/T3 timing markers in HttpTransport.SendBatchAsync

Temporary diagnostic for the Unity 6 Linux PlayMode regression. Tests
on Unity 6 Mono Linux take ~33 sec each vs ~1.8 sec on Unity 2021.3
Mono Linux for the same suite, same llvmpipe, same network endpoint.

T1 logs request start, T2 logs response received (continuation back on
the main thread), T3 logs batch deletion done. Comparing T1->T2 vs
T2->T3 gaps across Unity versions disambiguates whether the stall is
inside HttpClient.SendAsync (network/socket/TLS regression in Unity 6
Mono on Linux) or in the awaiter continuation (SyncContext dispatch
regression on Unity 6 Linux player loop).

To revert: drop this commit.

Author: ImmutableJeffrey

src/Packages/Audience/Runtime/Transport/HttpTransport.cs

@@ -95,8 +95,23 @@ internal async Task<bool> SendBatchAsync(CancellationToken ct = default)
                 request.Content = new StringContent(payload, Encoding.UTF8, "application/json");
 #endif
 
+                // Temporary diagnostic markers to measure where SendBatchAsync
+                // spends time on Unity 6 Linux PlayMode cells (where each test
+                // sits at the 30 sec WaitForLogEntry budget on Mono Linux but
+                // finishes in 1-2 sec on Unity 2021.3 Linux). T1 is request
+                // start, T2 is response received (continuation alive), T3 is
+                // batch deletion + state reset done. Comparing T1->T2 vs
+                // T2->T3 gaps across Unity versions disambiguates whether the
+                // stall is in HttpClient.SendAsync (network/socket) or in the
+                // continuation back onto Unity's main thread (SyncContext).
+                var __t1 = System.Diagnostics.Stopwatch.StartNew();
+                Log.Debug($"[HttpTransport][T1] SendAsync start url={_url} count={batch.Count}");
+
                 using var response = await _client.SendAsync(request, ct).ConfigureAwait(false);
 
+                var __t2Ms = __t1.Elapsed.TotalMilliseconds;
+                Log.Debug($"[HttpTransport][T2] SendAsync done t1->t2={__t2Ms:F1}ms status={(int)response.StatusCode}");
+
                 var statusCode = (int)response.StatusCode;
 
                 if (statusCode >= 200 && statusCode < 300)
@@ -113,6 +128,7 @@ internal async Task<bool> SendBatchAsync(CancellationToken ct = default)
                         NotifyError(AudienceErrorCode.ValidationRejected,
                             $"Batch partially rejected: {rejected} of {batch.Count} events dropped");
                     }
+                    Log.Debug($"[HttpTransport][T3] batch handled t1->t3={__t1.Elapsed.TotalMilliseconds:F1}ms (t2->t3={(__t1.Elapsed.TotalMilliseconds - __t2Ms):F1}ms)");
                 }
                 else if (statusCode == 429)
                 {