fix: validate and re-cap all inbound worker-port traffic (Codex round 1)

The host's message listener trusted the compile-time WorkerToHost shape on
traffic from a peer that runs model code: postMessage(null) threw in the
listener and crashed the host process; forged log/done messages bypassed
maxLogBytes/maxValueBytes (the worker-side LogBuffer and prepareValue cap
only honest flows); and the error-reply renegotiation re-echoed a forged
non-cloneable call id, throwing outside any catch.

Every inbound message now passes a runtime shape gate that validates and
REBUILDS it field by field (junk drops without a throw; call ids must be
numbers, so replies are always clone-plain; forged extra fields never ride
along). One host-side ledger bounds everything landing in logs — honest
port entries, forged ones, and stray pipe bytes — at the single documented
maxLogBytes, with the shared in-band truncation marker emitted host-side
when the ledger trips first; the completion value is re-capped host-side
through the same prepareValue (with exactly the truncation suffix as slack
so honest worker-capped values pass unchanged), and done error text is
bounded. Also folds the stray-capture budget into that shared ledger
(round-1 finding B: it was a second maxLogBytes on top of the documented
shared cap).

packages/code-runtime/code-runtime-worker/src/protocol.ts

@@ -63,3 +63,16 @@ export type WorkerToHost = CallMessage | LogMessage | DoneMessage
 export type ReplyMessage =
   | { type: 'reply'; id: number; ok: true; value: unknown }
   | { type: 'reply'; id: number; ok: false; message: string }
+
+/**
+ * The in-band marker entry text announcing that log capture stopped at the
+ * byte budget. Shared wire vocabulary: the worker's LogBuffer emits it when
+ * ITS budget exhausts, and the host emits the identical text when its own
+ * ledger drops an entry first (forged port traffic, stray pipe bytes) — so
+ * a truncated run reads the same however the cap was hit.
+ * @param maxBytes - the configured `maxLogBytes` the marker names.
+ * @returns the marker line.
+ */
+export function logTruncationMarker(maxBytes: number): string {
+  return `[dsh-code-runtime-worker] log capture truncated at ${maxBytes} bytes`
+}