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).

docs/WorkerFrameProtocol.md

@@ -29,6 +29,29 @@ default. A `max_frame_bytes` of 0 (an older gateway that never set the field)
 means "use the worker's built-in default". This keeps both ends framing to the
 same limit rather than depending on matched compile-time constants.
 
+Every worker-to-gateway frame must serialize within this limit, control replies
+included, so reply builders truncate to fit rather than emit a frame the writer
+will reject. `WorkerPipeSession` pre-sizes a `DrainEvents` reply below the
+negotiated maximum (less a fixed envelope/reply-wrapper reserve) and reports the
+truncation in the reply's `DiagnosticMessage`; the caller contract is to repeat
+`DrainEvents` until it returns an empty reply. Should a reply still overshoot,
+`MessageTooLarge` at the reply-write seam is answered with a small
+`InvalidRequest` reply for that correlation, not with session teardown — no
+diagnostics command may kill a session.
+
+An oversized *event* frame is the deliberate exception. Such an event is
+undeliverable end to end (the pipe maximum sits only the envelope-overhead
+reserve above the public gRPC cap), so the session faults: the worker logs the
+event's identity and sizes — never its value — writes a `WorkerFault` with
+category `ProtocolViolation` and command method `EventDrain`, then exits.
+Remediation is raising `MxGateway:Worker:MaxMessageBytes` for that workload.
+
+A per-frame rejection does not consume an envelope `sequence`. The writer stamps
+a candidate sequence, runs the empty-payload and size checks against the stamped
+envelope, and commits the counter only immediately before the stream write, so
+the sequences observed on the wire stay contiguous across rejections and an
+operator reading a pipe capture never sees a phantom gap.
+
 ## Envelope Validation
 
 `WorkerFrameReader` and `WorkerFrameWriter` validate each envelope against the