feat(worker): adopt negotiated frame max, bound drain, priority write scheduler (IPC-02/04 + WRK-04/07 worker half)

Worker half of the Wave 3 size/backpressure + write-ordering pass:

- IPC-02: the worker adopts GatewayHello.max_frame_bytes during the handshake
  (WorkerFrameProtocolOptions.AdoptNegotiatedMaxMessageBytes) instead of a
  hard-coded default; 0 keeps the default, a value above a 256 MiB ceiling is
  rejected. Reader and writer share the options instance, applied before the
  message loop.
- IPC-04: DrainEvents caps each reply at MaxDrainEventsPerReply (10_000) and
  treats max_events = 0 as that cap rather than 'drain the entire queue', so one
  diagnostic drain cannot pack a session-killing reply frame.
- WRK-04: WorkerFrameWriter stamps the envelope Sequence at the actual point of
  writing (under the write lock) instead of at envelope creation, so the on-wire
  order and the stamped sequence always agree under concurrent producers.
- WRK-07: the writer is now a cooperative priority scheduler — callers enqueue at
  Control or Event priority and the draining lock-holder writes all control
  frames before any event frame, so replies/faults/heartbeats jump ahead of an
  event backlog. Per-frame validation/size rejections fail only that frame; a
  stream write failure fails all queued frames.

Tests: monotonic gap-free sequence under concurrency, control-before-event
priority (gated stream), negotiated-max adoption, DrainEvents zero-bound.
Worker builds x86 only — verified on windev.

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

@@ -0,0 +1,17 @@
+namespace ZB.MOM.WW.MxGateway.Worker.Ipc;
+
+/// <summary>
+/// Relative scheduling priority for an outbound worker frame. The single writer task drains all
+/// pending <see cref="Control"/> frames before any <see cref="Event"/> frame, so a command reply,
+/// fault, heartbeat, or shutdown acknowledgement is not delayed behind a backlog of queued events
+/// (WRK-07). Priority only reorders the write; the frame sequence is stamped at actual write time,
+/// so the on-wire order and the envelope <c>Sequence</c> always agree (WRK-04).
+/// </summary>
+public enum WorkerFrameWritePriority
+{
+    /// <summary>Control-plane frame (hello, ready, command reply, heartbeat, fault, shutdown ack). Written ahead of events.</summary>
+    Control = 0,
+
+    /// <summary>Event frame. Written only when no control frame is pending.</summary>
+    Event = 1,
+}