feat(ipc): negotiate worker frame max, add gRPC headroom, bound DrainEvents (IPC-02/03/04 gateway half)

Proto foundation + gateway-side of the size/backpressure pass:

- IPC-02: add GatewayHello.max_frame_bytes (regen Generated/); gateway sends
  its negotiated worker-frame max in the handshake so the worker can adopt it
  instead of a hard-coded default. Worker read-half lands separately.
- IPC-03: give the pipe frame max envelope-overhead headroom above the public
  gRPC cap (WorkerFrameProtocolOptions.EnvelopeOverheadReserveBytes = 64 KiB;
  default Worker.MaxMessageBytes bumped to 16 MiB + reserve), cross-validate the
  headroom at startup, and pre-check command envelope size in WorkerClient so an
  oversized command fails only that correlation (ResourceExhausted) instead of
  faulting the whole session.
- IPC-04: reject DrainEvents max_events above a public ceiling in the request
  validator (worker per-reply cap lands with the worker half).

Docs: GatewayConfiguration, WorkerFrameProtocol, gateway.md.
Tests: headroom validation, DrainEvents bound, oversized-command per-command
failure (pipe-harness, verified on windev).

src/ZB.MOM.WW.MxGateway.Server/Workers/WorkerClientErrorCode.cs

@@ -12,4 +12,9 @@ public enum WorkerClientErrorCode
     GatewayShutdown,
     WriteFailed,
     PendingCommandLimitExceeded,
+
+    // The serialized command envelope exceeds the negotiated worker-frame maximum. Rejected at the
+    // enqueue boundary so only the offending command fails (mapped to ResourceExhausted) instead of
+    // the oversized frame reaching the write loop and faulting the whole session (IPC-03).
+    CommandTooLarge,
 }