docs(tracking): record the WRK-21 cluster as Done with windev evidence

Change-log row for 2026-08-07: what landed for WRK-21/WRK-28/WRK-23/IPC-30, why
IPC-23 stays In progress (proto-comment/doc wave pending), and the verification
evidence — macOS NonWindows build + validator tests, and the documented windev
path (scripts/ci/windev-worker-ci.ps1 -Mode test) at a256560: x86 Worker build
clean, Worker.Tests 367 passed / 0 failed / 11 skipped.

src/ZB.MOM.WW.MxGateway.Worker.Tests/MxAccess/MxAccessEventQueueTests.cs

@@ -300,9 +300,9 @@ public sealed class MxAccessEventQueueTests
         Assert.Equal(WorkerFaultCategory.MxaccessEventConversionFailed, queue.Fault?.Category);
     }
 
-    // Mirrors MxAccessEventQueue's per-event repeated-field allowance. Kept local (and asserted
-    // through the budgets below) so a change to the queue's charge shows up as a failing bound
-    // rather than silently loosening these tests.
+    // Mirrors MxAccessEventQueue's per-event repeated-field allowance. Kept local rather than made
+    // public on the queue: the byte-budget tests state their budgets in units of that charge, so a
+    // change to it should surface here as a failing bound instead of silently moving with the code.
     private const int RepeatedFieldOverheadBytes = 8;
 
     /// <summary>