perf(worker): memoize event frame size at enqueue; drain stops sizing under the STA's lock

docs/MxAccessWorkerInstanceDesign.md

@@ -541,7 +541,11 @@ is bounded on **two** axes because no diagnostics command may be session-fatal:
   maximum less a 64 KiB envelope/reply-wrapper reserve, and the size decision
   happens inside the event queue's lock, so an event is dequeued only once it is
   known to fit. An event that does not fit stays at the head of the queue and is
-  never lost.
+  never lost. Each event's serialized size is *measured* once at enqueue, outside
+  that lock, and stored beside it: the drain only compares memoized numbers, so a
+  large drain never walks messages under the lock the STA needs to enqueue the
+  next COM callback. The memoized size cannot go stale because an enqueued event
+  is never mutated again (WRK-11).
 
 Truncation is reported in the reply's existing `DiagnosticMessage`
 ("N events returned, M remain; repeat DrainEvents for the rest") rather than in a