fix(WRK-21,WRK-28,WRK-23,IPC-30): byte-budget the DrainEvents reply, stop size errors from killing sessions

WRK-21 — DrainEvents was bounded by event count only, so a byte-heavy queue
(large string/array MxValues) built a reply above the negotiated frame maximum:
the writer rejected the frame, the exception unwound the session, and the events
already dequeued were destroyed. The drain is now byte-budgeted inside the queue
lock, so an event is dequeued only once it is known to fit and one that does not
stays at the head. Truncation is reported through the reply's existing
DiagnosticMessage (no contract change); callers drain until an empty reply. Both
reply-write seams — the control-command path and ProcessCommandAsync — now catch
MessageTooLarge and answer the correlation with an InvalidRequest reply instead
of unwinding or faulting the session. Satisfies IPC-23 R1-R3.

WRK-28 — the 10,000 drain ceiling moves to GatewayContractInfo
.MaxDrainEventsPerCommand, referenced by both the gateway request validator and
the worker clamp, replacing a comment-only sync contract. C# const only; no
.proto change.

WRK-23 — WorkerFrameWriter now peek-stamps, validates, then commits the sequence
counter immediately before the stream write, so a per-frame rejection leaves no
phantom gap on the wire.

IPC-30 — an oversized event frame stays session-fatal (it is undeliverable end to
end and neither dropping nor synthesizing a replacement is allowed), but the
death is structured: the event's identity and sizes are logged (never its value),
a WorkerFault with category PROTOCOL_VIOLATION and command method EventDrain is
written, then the session exits as before.

Docs updated in the same change: MxAccessWorkerInstanceDesign.md (drain byte cap,
truncation contract, oversized-head behavior, oversized-event policy, no control
reply is session-fatal on size), WorkerFrameProtocol.md (reply pre-sizing,
non-fatal reply-size rule, oversized-event policy, rejected frames do not consume
sequence numbers), gateway.md (DrainEvents two-axis bound).

gateway.md

@@ -447,10 +447,14 @@ Optional diagnostics:
 - `Ping`
 - `GetSessionState`
 - `GetWorkerInfo`
-- `DrainEvents` — diagnostic; `max_events` is bounded (the gateway rejects requests
-  above a public ceiling, and the worker caps each reply at its own per-reply limit,
-  treating `max_events = 0` as "the default cap") so one drain cannot pack an
-  unbounded, session-killing reply frame.
+- `DrainEvents` — diagnostic; the reply is bounded on two axes so one drain cannot
+  pack an unbounded, session-killing reply frame. By **count**: the gateway rejects
+  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.
 - `ShutdownWorker`
 
 Do not compress MXAccess semantics into generic verbs too early. A command enum