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).

archreview/2026-07-12/remediation/30-contracts-ipc.md

@@ -12,14 +12,14 @@ All `path:line` citations were re-verified against the working tree at `4f5371f`
 
 | ID | Sev | Tier | Eff | Dep | Status | Title |
 |----|-----|------|-----|-----|--------|-------|
-| IPC-23 | Medium | P0 | S¹ | WRK-21 | Not started | DrainEvents bound is count-based only; byte-heavy queue still builds a session-killing reply frame (contract requirements here; fix mechanics in WRK-21) |
+| IPC-23 | Medium | P0 | S¹ | WRK-21 | In progress — mechanics landed with WRK-21; proto-comment/doc wave pending | DrainEvents bound is count-based only; byte-heavy queue still builds a session-killing reply frame (contract requirements here; fix mechanics in WRK-21) |
 | IPC-24 | Medium | P0 | S | — | Not started | CI's unconditional Java churn-revert masks real generated-code drift for message-level proto changes |
 | IPC-25 | Medium | P0 | M | — | Not started | Committed Go/Python worker bindings are stale at HEAD; no guard covers them |
 | IPC-26 | Low | P2 | S¹ | WRK-22 | Not started | Cancelled write leaves a ghost frame that is still written (contract requirement here; fix mechanics in WRK-22) |
 | IPC-27 | Low | P2 | S | — | Not started | Descriptor freshness test blind to enums, enum values, services/methods, and the Galaxy contract |
 | IPC-28 | Low | — | S | — | Not started | `docs/Grpc.md` omits the `CommandTooLarge` → `ResourceExhausted` mapping |
 | IPC-29 | Low | — | S | — | Not started | Worker writer priority scheduling and write-time sequence stamping undocumented in the frame-protocol doc |
-| IPC-30 | Low | P0 | M | WRK-21 (same file/batch) | Not started | Oversized worker→gateway event frame is session-fatal — make the death deliberate, structured, and diagnosable |
+| IPC-30 | Low | P0 | M | WRK-21 (same file/batch) | Done | Oversized worker→gateway event frame is session-fatal — make the death deliberate, structured, and diagnosable |
 | IPC-31 | Info | — | — | — | N/A | Gateway stamps sequence at creation, worker at write — accepted divergence; sequence is documented diagnostic-only (`gateway.md:328-330`); revisit only if sequence ever becomes load-bearing |
 | IPC-32 | Info | — | S | IPC-25 | Not started | `check-codegen.ps1` check labels miscounted (folded into the IPC-25 script edit) |