fix(audience): restore 40s step in HttpTransport backoff schedule

Reverses the schedule narrowing from 0e6c121. The doubled progression
(5s → 10s → 20s → 40s → 60s) gives one extra "soft" backoff step
before hitting the cap and matches the original bitshift formula's
natural shape. The 4× jump from 20s → 60s in the previous schedule
was asymmetric; this restores clean doubling at every step.

The plan document this conflicts with should be updated to match.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: ImmutableJeffrey

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

@@ -122,14 +122,15 @@ internal async Task<bool> SendBatchAsync(CancellationToken ct = default)
 
         /// <summary>
         /// Backoff delay in milliseconds based on consecutive failures.
-        /// Schedule per plan: 0 → 5s → 10s → 20s → 60s cap.
+        /// Schedule: 0 → 5s → 10s → 20s → 40s → 60s cap.
         /// </summary>
         internal int BackoffMs => _consecutiveFailures switch
         {
             <= 0 => 0,
             1 => 5_000,
             2 => 10_000,
             3 => 20_000,
+            4 => 40_000,
             _ => 60_000,
         };
 

src/Packages/Audience/Tests/Runtime/HttpTransportTests.cs

@@ -173,7 +173,7 @@ public async Task BackoffMs_EscalatesOnlyAfterWindowElapsed()
             using var transport = new HttpTransport(_store, "pk_imapik-test-key1",
                 handler: handler, getUtcNow: _getUtcNow);
 
-            // Schedule per plan: 5s → 10s → 20s → 60s cap (no 40s step).
+            // Schedule: 5s → 10s → 20s → 40s → 60s cap.
             // Each escalation requires the previous window to have elapsed.
             await transport.SendBatchAsync();
             Assert.AreEqual(5_000, transport.BackoffMs);
@@ -188,7 +188,11 @@ public async Task BackoffMs_EscalatesOnlyAfterWindowElapsed()
 
             Advance(20_001);
             await transport.SendBatchAsync();
-            Assert.AreEqual(60_000, transport.BackoffMs, "jumps to 60s cap after 20s");
+            Assert.AreEqual(40_000, transport.BackoffMs);
+
+            Advance(40_001);
+            await transport.SendBatchAsync();
+            Assert.AreEqual(60_000, transport.BackoffMs, "reaches 60s cap after 40s step");
 
             Advance(60_001);
             await transport.SendBatchAsync();