fix(agent): spool pending-bytes drift guard + WDP backstop fallback test

Codex findings on d8c84f41/a369ff99: (1) MarkUploaded now recomputes
_pendingBytes from the pending list when accounting drifts negative or
stays non-zero on an empty list — a silent negative would disarm the
spool-pressure trigger. (2) New test covers the DevicePrepCompletion
handler''s AwaitingDesktop fallback (stale/replayed timer without a
desktop fact): synthetic Hello timeout, park, and completion on the
later real desktop arrival. Third finding (HashingCappedReadStream
maxBytes:0) refuted — the 0=uncapped contract is implemented in the
same class and exercised by the heal-path tests.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/Agent/AutopilotMonitor.Agent.V2.Core/Transport/Telemetry/TelemetrySpool.cs

@@ -258,6 +258,18 @@ namespace AutopilotMonitor.Agent.V2.Core.Transport.Telemetry
                     _pendingBytes -= _pending.First.Value.ByteLength;
                     _pending.RemoveFirst();
                 }
+
+                // _pendingBytes drives the spool-pressure trigger: negative drift would
+                // silently disarm it, a stale positive on an empty list would pin it on.
+                // The list is authoritative — on drift, recompute from it and log loudly.
+                if (_pendingBytes < 0 || (_pending.Count == 0 && _pendingBytes != 0))
+                {
+                    long recomputed = 0;
+                    foreach (var entry in _pending) recomputed += entry.ByteLength;
+                    _logger?.Warning(
+                        $"TelemetrySpool: pending-bytes accounting drifted to {_pendingBytes} with {_pending.Count} pending items after MarkUploaded({upToItemIdInclusive}) — recomputed to {recomputed}.");
+                    _pendingBytes = recomputed;
+                }
             }
         }
 

src/Agent/AutopilotMonitor.DecisionCore.Tests/DevicePreparationEngineTests.cs

@@ -192,6 +192,33 @@ namespace AutopilotMonitor.DecisionCore.Tests
             Assert.Equal(SessionStage.Completed, completed.NewState.Stage);
         }
 
+        [Fact]
+        public void BackstopFired_WithoutDesktop_ParksInAwaitingDesktopUntilDesktopCompletes()
+        {
+            // The arming site (HandleDesktopArrivedV1) guarantees desktop-first, so this
+            // fires only on a stale/replayed timer — Dispatch routes DeadlineFired purely on
+            // the payload name, no armed-state check. The fallback must keep the handler
+            // total: synthetic Hello timeout recorded, session parks in AwaitingDesktop, and
+            // a later real desktop arrival still completes it.
+            var engine = new DecisionEngine();
+            var state = SeedWdpSession(engine);
+
+            var fired = engine.Reduce(state, DeadlineFired(20, T0.AddMinutes(39), DeadlineNames.DevicePrepCompletion));
+
+            Assert.Equal(SessionStage.AwaitingDesktop, fired.NewState.Stage);
+            Assert.NotNull(fired.NewState.HelloResolvedUtc);
+            Assert.Equal("Timeout", fired.NewState.HelloOutcome!.Value);
+            Assert.DoesNotContain(fired.NewState.Deadlines, d => d.Name == DeadlineNames.DevicePrepCompletion);
+
+            // Hello is satisfied (synthetically) — only the desktop is still missing.
+            var waiting = Assert.Single(CompletionWaitingEffects(fired));
+            Assert.Equal("desktop_arrival", waiting.Parameters!["missingPrerequisites"]);
+
+            var completed = engine.Reduce(fired.NewState,
+                MakeSignal(21, DecisionSignalKind.DesktopArrived, T0.AddMinutes(45)));
+            Assert.Equal(SessionStage.Finalizing, completed.NewState.Stage);
+        }
+
         [Fact]
         public void HelloResolved_BeforeBackstopFires_CompletesNormally()
         {