fix(IPC-33): bound the DrainEvents byte budget against the gRPC send cap, not just the pipe frame max

The worker budgeted a DrainEvents reply against the negotiated pipe frame
maximum less one 64 KiB reserve, but the gateway returns that same
MxCommandReply verbatim through Invoke, where MaxSendMessageSize applies. The
two limits were coupled only from below (pipe >= gRPC + reserve), so any
configuration the validator permitted with a wider gap let the worker pack a
reply the pipe carried and the gRPC layer then refused with ResourceExhausted --
after Drain had already removed the events from the worker queue, losing them to
both the caller and the event stream.

Option (a) from the finding: close the coupling at both ends.

- GatewayOptionsValidator adds the upper half of the headroom rule
  (Worker:MaxMessageBytes <= Protocol:MaxGrpcMessageBytes + 2 x reserve), with a
  message naming the DrainEvents budget as the reason and the two knobs to move.
- WorkerPipeSession.ResolveDrainReplyByteBudget subtracts the reserve twice --
  once for the pipe frame, once so the MxCommandReply wrapper clears the gRPC
  send cap -- keeping the MaxMessageBytes/2 floor that makes small frame maxima
  usable.

The two changes are load-bearing for each other: the validator band is what
makes "frame max - 2 x reserve" provably at or below the gRPC cap, and the
doubled subtraction is what makes the band sufficient.

Docs updated in the same change (GatewayConfiguration.md both option rows plus
the headroom prose, WorkerFrameProtocol.md drain budget, gateway.md DrainEvents
byte axis). Tests: new GatewayOptionsValidatorTests cases for the upper bound
(reject) and both edges of the band (accept); GatewayOptionsTests' override case
now moves both message-size knobs together, since the band makes moving one
alone illegal.

The worker half cannot be built on macOS; verified on windev separately.

gateway.md

@@ -567,9 +567,14 @@ Optional diagnostics:
   requests above the shared ceiling `GatewayContractInfo.MaxDrainEventsPerCommand`
   and the worker clamps to the same value, treating `max_events = 0` as "the default
   cap". By **bytes**: the worker sizes the reply while draining, against the
-  negotiated worker-frame maximum, so a byte-heavy queue truncates instead of
-  overshooting and events that do not fit stay queued. Truncation is reported in the
-  reply's `DiagnosticMessage`; callers drain iteratively until an empty reply.
+  negotiated worker-frame maximum less two envelope reserves, so a byte-heavy queue
+  truncates instead of overshooting and events that do not fit stay queued. The
+  reserve is subtracted twice because the reply has two boundaries to clear — the
+  pipe frame, then the public gRPC send cap it is returned through by `Invoke` — and
+  the validator keeps `Worker:MaxMessageBytes` within two reserves of
+  `Protocol:MaxGrpcMessageBytes` so that budget always fits both. Truncation is
+  reported in the reply's `DiagnosticMessage`; callers drain iteratively until an
+  empty reply.
 - `ShutdownWorker`
 
 Do not compress MXAccess semantics into generic verbs too early. A command enum