fix(worker): observe detached drain faults per NEXT-04 discipline

Both tasks the detached-lock-wait path starts and discards now carry the
file's fault-observing continuation, factored out of ObserveAbandonedFault as
ObserveFault so the idiom has one definition. DrainDetachedAsync swallows the
drain but its _writeLock.Release() sits in a finally outside that catch, so a
Release that ever throws — a SemaphoreFullException from some future
double-release regression — had no awaiter and would have surfaced on net48 as
TaskScheduler.UnobservedTaskException at finalization instead of an
attributable failure. Same for a throw out of OnDetachedLockWaitSettled.

Review's Minor (distinguishing a cancelled from a faulted lock wait before
tombstoning) is deliberately not taken: a faulted WaitAsync is unreachable
here — nothing disposes _writeLock — so the branch would be untestable new
logic whose only effect is internal state, the caller already receiving the
fault itself from the rethrow. Recorded as a comment at the site instead.

edited on macOS, windev verification pending (plan Task 11). Re-ran the net10
scratch harness over WorkerFrameWriter and the writer suite: 0 warnings,
31/31 pass.

src/ZB.MOM.WW.MxGateway.Worker/Ipc/WorkerFrameWriter.cs

@@ -172,6 +172,12 @@ public sealed class WorkerFrameWriter
                 // caller still observes cancellation while the frame reaches the wire (documented
                 // above). Rethrow from the wait rather than from the completion, so a claimed frame's
                 // canceller is not held behind the very write it is abandoning.
+                //
+                // The wait ending without the lock IS cancellation in every reachable case — nothing
+                // disposes _writeLock — so the tombstone's TrySetCanceled is honest. A hypothetical
+                // faulted wait would take this same path and label the frame cancelled instead of
+                // faulted; that is internal state only, since the await below rethrows the fault itself
+                // to the caller.
                 TombstoneIfUnclaimed(frame, cancellationToken);
                 await lockWait.ConfigureAwait(false);
             }
@@ -343,8 +349,23 @@ public sealed class WorkerFrameWriter
     /// <param name="frame">Frame whose completion may fault without an awaiter.</param>
     private static void ObserveAbandonedFault(PendingFrame frame)
     {
-        _ = frame.Completion.Task.ContinueWith(
+        ObserveFault(frame.Completion.Task);
-            task => _ = task.Exception,
+    }
+
+    /// <summary>
+    ///     Attaches the NEXT-04 fault-observing continuation to a task this writer starts and then
+    ///     discards. Every such task must carry one: with no awaiter, a fault would otherwise reach
+    ///     nobody and resurface as <see cref="TaskScheduler.UnobservedTaskException"/> at finalization
+    ///     — a detached, unattributable failure long after the code that caused it. The continuation
+    ///     runs only on the faulted path and only touches <see cref="Task.Exception"/>, so it can
+    ///     never fault itself; it runs inline because an already-faulted task would otherwise pay a
+    ///     scheduling hop to do nothing.
+    /// </summary>
+    /// <param name="task">Discarded task whose fault would otherwise go unobserved.</param>
+    private static void ObserveFault(Task task)
+    {
+        _ = task.ContinueWith(
+            faulted => _ = faulted.Exception,
             CancellationToken.None,
             TaskContinuationOptions.OnlyOnFaulted | TaskContinuationOptions.ExecuteSynchronously,
             TaskScheduler.Default);
@@ -367,11 +388,14 @@ public sealed class WorkerFrameWriter
     /// <param name="lockWait">Outstanding write-lock acquisition the caller has walked away from.</param>
     private void DetachLockWait(Task lockWait)
     {
-        _ = lockWait.ContinueWith(
+        // The continuation task is discarded, so it takes the NEXT-04 fault observer: nothing awaits
+        // it, and a throw out of OnDetachedLockWaitSettled would otherwise be an unobserved-task
+        // exception raised at finalization rather than an attributable failure here.
+        ObserveFault(lockWait.ContinueWith(
             OnDetachedLockWaitSettled,
             CancellationToken.None,
             TaskContinuationOptions.None,
-            TaskScheduler.Default);
+            TaskScheduler.Default));
     }
 
     /// <summary>
@@ -394,7 +418,11 @@ public sealed class WorkerFrameWriter
             return;
         }
 
-        _ = DrainDetachedAsync();
+        // Discarded, so it takes the NEXT-04 fault observer too. DrainDetachedAsync swallows the drain
+        // itself, but its release sits in a finally outside that catch: a Release that ever throws (a
+        // SemaphoreFullException from some future double-release regression, say) must fail somewhere
+        // attributable rather than at finalization.
+        ObserveFault(DrainDetachedAsync());
     }
 
     /// <summary>